Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball having an excellent initial velocity when being hit at a head speed of 50 m/sec and an improved initial velocity when being hit at a head speed of 40 m/sec. The present invention provides a golf ball comprising a spherical core and a cover covering the spherical core, wherein the golf ball has a rebound equivalence energy ratio (R 40 ) ranging from 65.50% to 99.0% at a deformation amount of 7%, and a rebound equivalence energy ratio (R 50 ) ranging from 20.0% to 70.0% at a deformation amount of 19%, in a compression test (measurement temperature: −70° C., compression speed: 30 mm/min) applying a load to the golf ball along a radial direction of the golf ball.

FIELD OF THE INVENTION

The present invention relates to a golf ball.

DESCRIPTION OF THE RELATED ART

As a method for increasing a flight distance of a golf ball on drivershots, for example, a method of utilizing a core having a hardnessdistribution in which the hardness increases from the core centertowards the core surface, is known. A core having such a hardnessdistribution shows a high launch angle and a low spin rate. A golf ballshowing a high launch angle and a low spin rate travels a great flightdistance. As a golf ball comprising a core having such a hardnessdistribution, for example, a golf ball comprising a spherical coreformed from a rubber composition containing a base rubber, aco-crosslinking agent, a crosslinking initiator, and a salt of acarboxylic acid having 1 to 13 carbon atoms, has been proposed (refer toclaim 1 of Japanese Patent Publication No. 2013-027691 A).

The rubber composition used for a core of a golf ball usually containszinc acrylate as a co-crosslinking agent. A technology for improvingzinc acrylate has been proposed. For example, Japanese PatentPublication No. 2003-12600 A discloses a zinc acrylate compositioncontaining zinc acrylate and an anionic surfactant, wherein in the zincacrylate, particles having a particle size of 300 μm or more measured bya dry method is included in a ratio of 20 mass % or less, a particlesize measured by a dry method has a median value ranging from 10 to 300μm, and a ratio of the median value (A) of the particle size measured bya dry method to a median value (B) of a particle size measured by a wetmethod exceeds 2 (refer to claim 1 of Japanese Patent Publication No.2003-12600 A). Japanese Patent Publication No. 2004-161640 A discloses aproduction method of zinc acrylate, comprising dispersing zinc oxide inany one of an aliphatic hydrocarbon solvent, a mixed solvent containingan aliphatic hydrocarbon solvent and an aromatic hydrocarbon solvent, ora mixed solvent containing an aromatic hydrocarbon solvent and analcohol, and reacting acrylic acid with zinc oxide in the solvent (referto claim 1 of Japanese Patent Publication No. 2004-161640 A). JapanesePatent Publication No. S59-21640 A discloses a production method of zincacrylate, comprising reacting acrylic acid with a zinc compound, andthen adding stearic acid into the reaction liquid (refer to claims ofJapanese Patent Publication No. S59-21640 A).

In addition, a method of adding a filler such as an organic short fiber,a metal and a clay mineral, in addition to a resin component, into agolf ball constituent member to improve the golf ball performance, hasalso been proposed. For example, Japanese Patent Publication No.2004-504900 A discloses a golf ball comprising a nanocomposite material,wherein the nanocomposite material is formed from a polymer having astructure in which inorganic material particles are reacted andsubstantially evenly dispersed, and wherein each of the particles has alargest particle size that is about 1 μm or less and that is at least anorder of magnitude greater than such particle's smallest particle size(refer to claim 1 of Japanese Patent Publication No. 2004-504900 A).Japanese Patent Publications No. 2006-346014 A, No. 2009-178447 A, and2009-254750 A disclose a golf ball comprising a cover containing acation-treated layered silicate, a (meth)acrylic polymer modifiedsilicate or an organically modified layered silicate (refer to claim 5of Japanese Patent Publication No. 2006-346014 A, claim 1 of JapanesePatent Publication No. 2009-178447 A, and claim 1 of Japanese PatentPublication 2009-254750 A).

As a method of evaluating rebound characteristics of a golf ball, forexample, a method of evaluating rebound characteristics of an elasticbody has been proposed. In this method, a compression test applying aload to a ball along a radial direction of the ball is performed, andbased on the difference between the deformation state of the ball in thecompression test and the deformation state of the ball in an actualhitting phenomenon, an absorption energy and a discharge energy arecalculated according to load-deflection curves obtained when applyingthe load and when removing the load in the compression test, and thenconverted into kinetic energies before and after the collision,respectively, to obtain coefficient of restitution of the ball (refer toclaim 1 of Japanese Patent Publication No. 2000-121522 A).

SUMMARY OF THE INVENTION

As a method for improving a flight performance of a golf ball, there isa method of utilizing a core or cover having high resilience. Theoptimum resilience of a golf ball varies depending on the head speed forhitting the ball. Therefore, the resilience performance of a golf ballshould be adjusted according to the head speed for hitting the ball.

The present invention has been achieved in view of the above problems.An object of the present invention is to provide a golf ball having anexcellent initial velocity when being hit at a head speed of 50 m/sec aswell as an improved initial velocity when being hit at a head speed of40 m/sec.

The present invention provides a golf ball comprising a spherical coreand a cover covering the spherical core, wherein the golf ball has arebound equivalence energy ratio (R₄₀) ranging from 65.50% to 99.0% at adeformation amount of 7%, and a rebound equivalence energy ratio (R₅₀)ranging from 20.0% to 70.0% at a deformation amount of 19%, in acompression test (measurement temperature: −70° C., compression speed:30 mm/min) applying a load to the golf ball along a radial direction ofthe golf ball. If the rebound equivalence energy ratio (R₄₀) and therebound equivalence energy ratio (R₅₀) fall within the above ranges, theresultant golf ball shows small hysteresis loss and thus has highresilience when being hit at a head speed of both 50 m/sec and 40 m/sec.

The golf ball according to the present invention exhibits excellentresilience performance when being hit both at a head speed of 50 m/secand at a head speed of 40 m/sec, and exhibits particularly excellentresilience performance when being hit at a head speed of 40 m/sec.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ball of oneembodiment according to the present invention;

FIG. 2 is a graph showing a hardness distribution of a core;

FIG. 3 is a graph showing a hardness distribution of a core;

FIG. 4 is a graph showing a hardness distribution of a core;

FIG. 5 is a graph showing a hardness distribution of a core;

FIG. 6 is a graph showing a hardness distribution of a core;

FIG. 7 is a graph showing a hardness distribution of a core;

FIG. 8 is a graph showing a hardness distribution of a core;

FIG. 9 is a graph showing a hardness distribution of a core;

FIG. 10 is a graph showing a hardness distribution of a core;

FIG. 11 is a graph showing a hardness distribution of a core; and

FIG. 12 is a graph showing an example of a force-deflection curve in arebound equivalence energy measurement.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a golf ball comprising a spherical coreand a cover covering the spherical core, wherein the golf ball has arebound equivalence energy ratio (R₄₀) ranging from 65.50% to 99.0% at adeformation amount of 7%, and a rebound equivalence energy ratio (R₅₀)ranging from 20.0% to 70.0% at a deformation amount of 19%, in acompression test (measurement temperature: −70° C., compression speed:30 mm/min) applying a load to the golf ball along a radial direction ofthe golf ball.

The conditions of measurement temperature: −70° C., compression speed:30 mm/min and deformation amount: 7% in the compression test areequivalent to the compression conditions when a golf ball is hit with adriver at a head speed of 40 m/sec. In addition, the conditions ofmeasurement temperature: −70° C., compression speed: 30 mm/min anddeformation amount: 19% in the compression test are equivalent to thecompression conditions when a golf ball is hit with a driver at a headspeed of 50 m/sec. Therefore, if the rebound equivalence energy ratio(R₄₀) and the rebound equivalence energy ratio (R₅₀) fall within theabove ranges, the resultant golf ball shows small hysteresis loss andthus has high resilience when being hit at a head speed of both 50 m/secand 40 m/sec.

The rebound equivalence energy ratio (R₄₀) is preferably 65.50% or more,more preferably 65.7% or more, even more preferably 65.8% or more. Ifthe rebound equivalence energy ratio (R₄₀) is 65.50% or more, theresilience of the golf ball when being hit at a head speed of 40 m/secimproves. The upper limit of the rebound equivalence energy ratio (R₄₀)is not particularly limited, and is preferably 99.0%, more preferably85.0%, even more preferably 70.0%. If the rebound equivalence energyratio (R₄₀) is 99.0% or less, the golf ball has better durability.

The rebound equivalence energy (E₄₀) at a deformation amount of 7% ispreferably 3.60 N·m or more, more preferably 3.70 N·m or more, even morepreferably 3.82 N·m or more. If the rebound equivalence energy (E₄₀) is3.60 N·m or more, the resilience of the golf ball when being hit at ahead speed of 40 m/sec improves. The upper limit of the reboundequivalence energy (E₄₀) is not particularly limited, and is preferably10.0 N·m, more preferably 9.0 N·m, even more preferably 8.0 N·m. If therebound equivalence energy (E₄₀) is 10.0 N·m or less, the golf ball hasbetter durability.

The total applied energy (E_(40total)) at a deformation amount of 7% ispreferably 5.0 N·m or more, more preferably 5.3 N·m or more, even morepreferably 5.6 N·m or more. If the total applied energy (E_(40total)) is5.0 N·m or more, the resilience of the golf ball when being hit at ahead speed of 40 m/sec improves. The upper limit of the total appliedenergy (E_(40total)) is not particularly limited, and is preferably 15.0N·m, more preferably 13.0 N·m, even more preferably 11.0 N·m. If thetotal applied energy (E_(40total)) is 15.0 N·m or less, the golf ballhas better durability. The total applied energy (E_(40total)) is a totalenergy which is necessary to deform the golf ball in a compression ratioof up to 7%.

The rebound equivalence energy ratio (R₅₀) is preferably 20.0% or more,more preferably 30.0% or more, even more preferably 40.0% or more. Ifthe rebound equivalence energy ratio (R₅₀) is 20.0% or more, theresilience of the golf ball when being hit at a head speed of 50 m/secimproves. The upper limit of the rebound equivalence energy ratio (R₅₀)is not particularly limited, and is preferably 70.0%, more preferably65.0%, even more preferably 61.0%. If the rebound equivalence energyratio (R₅₀) is 70.0% or less, the golf ball has better durability.

The rebound equivalence energy (E₅₀) at a deformation amount of 19% ispreferably 5 N·m or more, more preferably 10 N·m or more, even morepreferably 18 N·m or more. If the rebound equivalence energy (E₅₀) is 5N·m or more, the resilience of the golf ball when being hit at a headspeed of 50 m/sec improves. The upper limit of the rebound equivalenceenergy (E₅₀) is not particularly limited, and is preferably 50 N·m orless, more preferably 40 N·m or less, even more preferably 25 N·m orless. If the rebound equivalence energy (E₅₀) is 50 N·m or less, thegolf ball has better durability.

The total applied energy (E_(50total)) at a deformation amount of 19% ispreferably 25 N·m or more, more preferably 35 N·m or more, even morepreferably 55 N·m or more. If the total applied energy (E_(50total)) is25 N·m or more, the resilience of the golf ball when being hit at a headspeed of 50 m/sec improves. The upper limit of the total applied energy(E_(50total)) is not particularly limited, and is preferably 80 N·m orless, more preferably or less, even more preferably 65 N·m or less. Ifthe total applied energy (E_(50total)) is 80 N·m or less, the golf ballhas better durability. The total applied energy (E_(50total)) is a totalenergy which is necessary to deform the golf ball in a compression ratioof up to 19%.

The difference (R₄₀-R₅₀) between the rebound equivalence energy ratio(R₄₀) and the rebound equivalence energy ratio (R₅₀) is preferably 1% ormore, more preferably 2% or more, even more preferably 3% or more. Ifthe difference (R₄₀-R₅₀) is 1% or more, the resilience performance ofthe golf ball when being hit at a head speed of 40 m/sec is better.Therefore, both the resilience performance of the golf ball when beinghit at a head speed of 50 m/sec and the resilience performance of thegolf ball when being hit at a head speed of 40 m/sec are better. Inparticular, the resilience performance of the golf ball when being hitat a head speed of 40 m/sec is further enhanced.

The golf ball preferably has a diameter ranging from 40 mm to 45 mm. Inlight of satisfying the regulation of US Golf Association (USGA), thediameter is most preferably 42.67 mm or more. In light of prevention ofair resistance, the diameter is more preferably 44 mm or less, and mostpreferably 42.80 mm or less. In addition, the golf ball preferably has amass of 40 g or more and 50 g or less. In light of obtaining greaterinertia, the mass is more preferably 44 g or more, and most preferably45.00 g or more. In light of satisfying the regulation of USGA, the massis most preferably 45.93 g or less.

When the golf ball has a diameter in a range from 40 mm to 45 mm, thecompression deformation amount (shrinking amount along the compressiondirection) of the golf ball when applying a load from an initial load of98 N to a final load of 1275 N to the golf ball is preferably 2.0 mm ormore, more preferably 2.2 mm or more, and is preferably 4.0 mm or less,more preferably 3.5 mm or less. If the compression deformation amount is2.0 mm or more, the golf ball does not become excessively hard, and thushas better shot feeling. On the other hand, if the compressiondeformation amount is 4.0 mm or less, the golf ball has betterresilience.

[Spherical Core]

The spherical core may have a single-layered construction or amulti-layered construction composed of at least two layers. Thespherical core preferably has a single-layered construction. Unlike themulti-layered spherical core, the single-layered spherical core does nothave an energy loss at the interface of the multi-layered constructionwhen being hit, and thus has better resilience. The spherical core ispreferably formed from a rubber composition.

The center hardness Ho of the spherical core is preferably 35 or more,more preferably 40 or more, even more preferably 45 or more in Shore Chardness. If the spherical core has a center hardness Ho of 35 or morein Shore C hardness, the spherical core does not become excessivelysoft, and thus has better resilience. In addition, the center hardnessHo of the spherical core is preferably 65 or less, more preferably 62 orless, even more preferably 61 or less in Shore C hardness. If thespherical core has a center hardness Ho of 65 or less in Shore Chardness, the spherical core does not become excessively hard, and thushas better shot feeling.

The surface hardness Hs of the spherical core is preferably 75 or more,more preferably 77 or more, even more preferably 80 or more, and ispreferably 95 or less, more preferably 93 or less, even more preferably91 or less in Shore C hardness. If the spherical core has a surfacehardness of 75 or more in Shore C hardness, the spherical core does notbecome excessively soft, and thus has better resilience. In addition, ifthe spherical core has a surface hardness of 95 or less in Shore Chardness, the spherical core does not become excessively hard, and thushas better shot feeling.

The hardness difference (Hs—Ho) between the surface hardness Hs and thecenter hardness Ho of the spherical core is preferably 15 or more, morepreferably 17 or more, even more preferably 20 or more, and ispreferably 40 or less, more preferably 38 or less, even more preferably35 or less in Shore C hardness. If the hardness difference is large, thegolf ball showing a higher launch angle and a lower spin rate, therebytravelling a greater flight distance can be obtained.

If a hardness is measured in the spherical core of the golf ballaccording to the present invention at nine points, including a corecenter and a core surface, obtained by dividing a radius of thespherical core into equal parts having 12.5% intervals therebetween, andis plotted against a distance from the core center, the coefficient ofdetermination R² of a linear approximation curve obtained from a leastsquare method is preferably 0.90 or more. If R² is 0.90 or more, thelinearity of the core hardness distribution is enhanced, and thus thespin rate on driver shots is lowered, resulting in the enhanced flightdistance performance.

The hardness of the spherical core is Shore C hardness measured in thespherical core at nine points, including a core center and a coresurface, obtained by dividing a radius of the spherical core into equalparts having 12.5% intervals therebetween. That is, Shore C hardness ismeasured at nine points, namely at a distance of 0% (core center),12.5%, 25%, 37.5%, 50%, 62.5%, 75%, 87.5% and 100% (core surface) fromthe core center. Next, the measurement results are plotted to make agraph having the Shore C hardness measured above as a vertical axis andthe distance (%) from the core center as a horizontal axis. In thepresent invention, R² of a linear approximation curve obtained from thisgraph by a least square method is preferably 0.90 or more. R² of thelinear approximation curve obtained from the least square methodindicates the linearity of the obtained plot. In the present invention,R² of 0.90 or more means that the spherical core has an approximatelylinear hardness distribution. If the spherical core having anapproximately linear hardness distribution is used, the resultant golfball has a lowered spin rate on driver shots. As a result, the golf balltravels a greater flight distance on driver shots. R² of the linearapproximation curve is more preferably 0.92 or more. A higher linearityprovides a greater flight distance on driver shots.

R² of the linear approximation curve is a parameter showing thedeviation between the hardness value calculated from the approximationcurve and the measured value. The value of R² nearer to 1 means asmaller deviation. When the linear approximation curve is expressed bythe Formula (1), R² is calculated according to Formulae (2) to (4).

$\begin{matrix}{{f(x)} = {{a\; x} + b}} & (1) \\{R^{2} = {1 - \frac{{\Sigma_{i}( {y_{i} - f_{i}} )}^{2}}{{\Sigma_{i}( {y_{i} - \overset{\_}{y}} )}^{2}}}} & (2) \\{f_{i} = {{a\; x_{i}} + b}} & (3) \\{\overset{\_}{y} = {\frac{1}{9}{\sum_{i = 1}^{9}y_{i}}}} & (4)\end{matrix}$

The diameter of the spherical core is preferably 34.8 mm or more, morepreferably 36.8 mm or more, even more preferably 38.8 mm or more, and ispreferably 42.2 mm or less, more preferably 41.8 mm or less, even morepreferably 41.2 mm or less, most preferably 40.8 mm or less. If thespherical core has a diameter of 34.8 mm or more, the thickness of thecover does not become excessively thick, and thus the resilience of thegolf ball is better. On the other hand, if the spherical core has adiameter of 42.2 mm or less, the cover does not become excessively thin,and thus the cover functions better.

When the spherical core has a diameter in a range from 34.8 mm to 42.2mm, the compression deformation amount (shrinking amount along thecompression direction) of the spherical core when applying a load froman initial load of 98 N to a final load of 1275 N to the spherical coreis preferably 2.0 mm or more, more preferably 2.8 mm or more, and ispreferably 6.0 mm or less, more preferably 5.0 mm or less. If thecompression deformation amount is 2.0 mm or more, the shot feeling isbetter, and if the compression deformation amount is 6.0 mm or less, theresilience is better.

[Cover]

The cover may be single-layered or may be composed of at least twolayers. It is preferred that at least one layer of the cover is formedfrom a first resin composition.

The slab hardness of the first resin composition is preferably 35 ormore, more preferably 40 or more, even more preferably 45 or more, andis preferably 65 or less, more preferably 60 or less, even morepreferably 56 or less in Shore D hardness. If the slab hardness is 35 ormore, the golf ball has better resilience since the golf ball deforms alittle when being hit and the hitting energy can be efficientlyconverted to accelerate the golf ball. In addition, if the slab hardnessis 65 or less, the golf ball has better shot feeling since the impactapplied to the golf ball when hitting the golf ball can be suppressed.

The bending stiffness (M₃₋₁₂) of the first resin composition ispreferably 500 kgf/cm² (49.0 MPa) or more, more preferably 600 kgf/cm²(58.8 MPa) or more, even more preferably 700 kgf/cm² (68.6 MPa) or more,most preferably 1000 kgf/cm² (98.1 MPa) or more, and is preferably 6000kgf/cm² (588 MPa) or less, more preferably 5500 kgf/cm² (539 MPa) orless, even more preferably 5000 kgf/cm² (490 MPa) or less. If thebending stiffness is 500 kgf/cm² or more, the golf ball has betterresilience since the golf ball deforms a little when being hit and thehitting energy can be efficiently converted to accelerate the golf ball.In addition, if the bending stiffness is 6000 kgf/cm² or less, the golfball has better shot feeling since the flexibility is better and theimpact applied to the golf ball when hitting the golf ball can besuppressed.

The ratio (slab hardness (Shore D)/bending stiffness (kgf/cm²)) of theslab hardness to the bending stiffness of the first resin composition ispreferably 0.003 or more, more preferably 0.005 or more, even morepreferably 0.01 or more, and is preferably 0.05 or less, more preferably0.045 or less, even more preferably 0.043 or less. If the ratio is 0.003or more, the golf ball has better shot feeling since the flexibility isbetter and the impact applied to the golf ball when hitting the golfball can be suppressed. In addition, if the ratio is 0.05 or less, thegolf ball has better resilience since the golf ball deforms a littlewhen being hit and the hitting energy can be efficiently converted toaccelerate the golf ball.

The thickness of the cover is preferably 0.3 mm or more, more preferably0.5 mm or more, even more preferably 0.8 mm or more, and is preferably4.0 mm or less, more preferably 3.0 mm or less, even more preferably 2.0mm or less. If the cover has a thickness of 0.3 mm or more, the coverfunctions better, and if the cover has a thickness of 4.0 mm or less,the core has a relatively large diameter, and thus the golf ball hasbetter resilience.

The cover of the golf ball is preferably composed of at least twolayers. If the cover includes at least two layers, properties of thegolf ball can be easily controlled. When the cover includes at least twolayers, it is preferred that the outermost cover is formed from a secondresin composition, and at least one layer of the cover other than theoutermost cover is formed from the first resin composition.

When the cover includes at least two layers, the thickness of the coverformed from the first resin composition is preferably 0.3 mm or more,more preferably 0.5 mm or more, even more preferably 0.8 mm or more, andis preferably 2.0 mm or less, more preferably 1.5 mm or less, even morepreferably 1.2 mm or less. If the cover formed from the first resincomposition has a thickness of 0.3 mm or more, the cover functionsbetter, and if the cover formed from the first resin composition has athickness of 2.0 mm or less, the core has a relatively large diameter,and thus the golf ball has better resilience.

The slab hardness of the second resin composition is preferably 10 ormore, more preferably 15 or more, even more preferably 20 or more, andis preferably 45 or less, more preferably 40 or less, even morepreferably 35 or less in Shore D hardness. If the outermost cover has aslab hardness of 10 or more, the golf ball has better resilience sincethe golf ball deforms a little when being hit and the hitting energy canbe efficiently converted to accelerate the golf ball. In addition, ifthe outermost cover has a slab hardness of 45 or less, the golf ball hasbetter shot feeling since the impact applied to the golf ball whenhitting the golf ball can be suppressed.

The thickness of the outermost cover is preferably 0.1 mm or more, morepreferably 0.2 mm or more, even more preferably 0.3 mm or more, and ispreferably 1.0 mm or less, more preferably 0.8 mm or less, even morepreferably 0.6 mm or less. If the outermost cover has a thickness of 0.1mm or more, the cover functions better, and if the outermost cover has athickness of 1.0 mm or less, the core has a relatively large diameter,and thus the golf ball has better resilience.

The construction of the golf ball according to the present invention isnot particularly limited, as long as the golf ball comprises a sphericalcore and a cover covering the spherical core. Examples of the golf ballinclude: a two-piece golf ball having a single-layered spherical coreand a single-layered cover disposed around the spherical core; athree-piece golf ball having a dual-layered spherical core and asingle-layered cover disposed around the spherical core; a three-piecegolf ball having a single-layered spherical core, an inner coverdisposed around the spherical core, and an outer cover disposed aroundthe inner cover; a four-piece golf ball having a dual-layered sphericalcore, an inner cover disposed around the spherical core, and an outercover disposed around the inner cover; a golf ball having at least fourpieces, i.e. a single-layered spherical core and at least three coverlayers disposed around the spherical core; and a golf ball having atleast five pieces, i.e. a dual-layered spherical core and at least threecover layers disposed around the spherical core. The present inventioncan be suitably applied to any one of the above golf balls.

It is preferred that the golf ball according to the present inventionhas a single-layered core, an inner cover and an outermost cover, and atleast one layer of the inner cover is formed from the first resincomposition.

FIG. 1 is a partially cutaway cross-sectional view of a golf ball 1 ofone embodiment according to the present invention. The golf ball 1 has aspherical core 2, an inner cover 3 covering the spherical core 2, and anoutermost cover 4 covering the inner cover 3. A plurality of dimples 41are formed on the surface of the outermost cover 4. Other portions thandimples 41 on the surface of the golf ball 1 are land 42. The golf ball1 is provided with a paint layer and a mark layer outside the outermostcover 4, but these layers are not depicted.

[Core Material]

A conventional rubber composition (hereinafter, sometimes merelyreferred to as “rubber composition”), for example, a rubber compositioncontaining (a) a base rubber, (b) a co-crosslinking agent and (c) acrosslinking initiator, can be used to form the spherical core.

((a) Base Rubber)

As (a) the base rubber, a natural rubber and/or a synthetic rubber canbe used. For example, a polybutadiene rubber, a natural rubber, apolyisoprene rubber, a styrene polybutadiene rubber, anethylene-propylene-diene rubber (EPDM), or the like can be used. Theserubbers may be used solely, or at least two of these rubbers may be usedin combination. Among them, typically preferred is a highcis-polybutadiene having a cis-1,4-bond in a proportion of 40 mass % ormore, preferably 80 mass % or more, more preferably 90 mass % or more inview of its superior resilience.

((b) Co-Crosslinking Agent)

(b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof is blended as a co-crosslinking agent inthe rubber composition and has an action of crosslinking a rubbermolecule by graft polymerization to a base rubber molecular chain.Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms preferably include acrylic acid, methacrylic acid, fumaric acid,maleic acid and crotonic acid. Among them, acrylic acid and methacrylicacid are preferred.

Examples of the metal ion constituting the metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include: amonovalent metal ion such as sodium, potassium and lithium; a divalentmetal ion such as magnesium, calcium, zinc, barium and cadmium; atrivalent metal ion such as aluminum; and other metal ion such as tinand zirconium. The above metal component may be used solely or as amixture of at least two of them. Among them, the divalent metal ion suchas magnesium, calcium, zinc, barium and cadmium is preferably used asthe metal component. This is because if the divalent metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms is used, ametal crosslinking easily generates between the rubber molecules.Especially, as the divalent metal salt, a zinc salt is preferable, andzinc acrylate is more preferable because zinc acrylate enhances theresilience of the obtained golf ball. The α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or the metal salt thereof may beused solely or as a mixture of at least two of them.

The amount of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof in the rubber composition ispreferably 15 parts by mass or more, more preferably 20 parts by mass ormore, and is preferably 50 parts by mass or less, more preferably 45parts by mass or less, even more preferably 35 parts by mass or less,with respect to 100 parts by mass of (a) the base rubber. If the amountof (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof is less than 15 parts by mass, the amountof (c) the crosslinking initiator which will be described later must beincreased such that the constituent member formed from the rubbercomposition has an appropriate hardness, which tends to lower theresilience of the golf ball. On the other hand, if the amount of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof exceeds 50 parts by mass, the constituent memberformed from the rubber composition becomes so hard that the shot feelingof the golf ball may be lowered.

((b1) Co-Crosslinking Agent Powder)

(b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof is preferably (b1) a powder of anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof (hereinafter, sometimes referred to as “(b1)co-crosslinking agent powder”).

The mode particle size of (b1) the co-crosslinking agent powder ispreferably larger than 10 μm and at least 50 μm. The mode particle sizeis a particle size having maximum value (most frequent value) in thevolume based frequency distribution graph. The mode particle size ispreferably larger than 10 μm, more preferably 13 μm or more, even morepreferably 15 μm or more. In addition, the mode particle size ispreferably 50 μm or less, more preferably 45 μm or less, even morepreferably 40 μm or less. If the mode particle size falls within theabove range, (b1) the co-crosslinking agent powder has betterkneadability, and thus the resultant golf ball constituent member(especially when it is a spherical core) has a greater degree of theouter-hard and inner-soft structure. As a result, the obtained golf ballhas a low spin rate on driver shots and hence travels a greater flightdistance. Since (b1) the co-crosslinking agent powder has a largeparticle size, (b1) the co-crosslinking agent powder remains asparticles in the formed core. In other words, the spherical coreincludes the particles of the α,β-unsaturated carboxylic acid having 3to 8 carbon atoms and/or the metal salt thereof.

In (b1) the co-crosslinking agent powder, the volume ratio (V₆₋₃₀₀) ofthe particles having a particle size in a range from 6 μm to 300 μm ispreferably 60% or more. The volume ratio (V₆₋₃₀₀) is a value obtained bysubtracting a cumulative volume ratio V % (6 μm) at the particle size of6 μm from a cumulative volume ratio V % (300 μm) at the particle size of300 μm in the volume based cumulative distribution graph (the fineparticle size side is 0%, and the coarse particle size side is 100%).The volume ratio (V₆₋₃₀₀) is more preferably 65% or more. If the volumeratio (V₆₋₃₀₀) is less than 60%, the ratio of the fine particles havinga particle size smaller than 6 μm or the coarse particles having aparticle size larger than 300 μm increases. As a result, the resultantrubber composition may not be kneaded uniformly. In addition, when thespherical core is formed from the rubber composition, the outer-hard andinner-soft degree thereof may be lowered.

The d10 of (b1) the co-crosslinking agent powder is preferably 6 μm ormore. The d10 is a particle size (μm) whose cumulative volume ratio V %is 10% in the volume based cumulative distribution graph. The d10 ispreferably 6.5 μm or more. If the d10 is less than 6 μm, the ratio ofthe fine particles having a particle size smaller than 6 μm increases,and when the spherical core is formed from the rubber composition, theouter-hard and inner-soft degree thereof may be lowered. The upper limitof the d10 is not particularly limited, and is preferably 15 μm, morepreferably 12 μm.

In (b1) the co-crosslinking agent powder, the volume ratio (V₀₋₂₀₀) ofthe particles having a particle size of 200 μm or less is preferably 75%or more. The volume ratio (V₀₋₂₀₀) is preferably 75% or more, morepreferably 76% or more. If the volume ratio (V₀₋₂₀₀) is 75% or more, theratio of the coarse particles having a particle size larger than 200 μmdecreases, and thus it becomes easier to knead the rubber compositionuniformly. The volume ratio (V₀₋₂₀₀) is not particularly limited, and ispreferably 98% or less, more preferably 95% or less.

The specific surface area of (b1) the co-crosslinking agent powder ispreferably 0.1 m²/g or more, more preferably 0.2 m²/g or more, even morepreferably 0.25 m²/g or more, and is preferably 1.5 m²/g or less, morepreferably 1.0 m²/g or less, even more preferably 0.8 m²/g or less. Ifthe specific surface area falls within the above range, (b1) theco-crosslinking agent powder has better kneadability, and thus theresultant golf ball constituent member (especially when it is aspherical core) has a greater degree of the outer-hard and inner-softstructure.

The mode particle size, d10, volume ratio and specific surface area aremeasured or calculated by the following methods. Specifically, a drypowder sample is set into a dry-type unit of a laser diffractionparticle size analyzer (type: LMS-2000e, available from SeishinEnterprise Co., Ltd.), the refractive index of the sample is set as1.52, and then measurement is conducted. Based on the obtained volumebased frequency distribution graph and the cumulative distributiongraph, the mode particle size and the volume ratio of the particles arecalculated.

Sometimes, (b1) the crosslinking agent powder is treated with a fattyacid and/or a salt thereof to improve the proccessability thereof in therubber composition. In this case, it is preferred that (b1) thecrosslinking agent powder before the treatment with the fatty acidand/or the salt thereof satisfies the above mentioned requirements.

(b1) The crosslinking agent powder may be obtained by pulverizing andclassifying the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or the metal salt thereof. The pulverization method is notparticularly limited, and examples thereof include a pulverizationmethod using a jet mill, ball mill or stamp mill. In addition, examplesof the classification method include a classification method based on anair flow and a classification method based on a sieve.

It is preferred that (b1) the crosslinking agent powder is coated withthe fatty acid and/or the salt thereof. The fatty acid is notparticularly limited, and a fatty acid having 10 to 30 carbon atoms ispreferable, a fatty acid having 10 to 20 carbon atoms is morepreferable. The fatty acid may be a saturated fatty acid or anunsaturated fatty acid. Suitable examples of the fatty acid and/or thesalt thereof include stearic acid, oleic acid, zinc stearate, and zincoleate.

The amount of the fatty acid and/or the salt thereof in (b1) thecrosslinking agent powder treated with the fatty acid and/or the saltthereof is preferably 1 mass % or more, more preferably 5 mass % ormore, even more preferably 10 mass % or more, and is preferably 20 mass% or less, more preferably 15 mass % or less.

((c) Crosslinking Initiator)

(c) The crosslinking initiator is blended to crosslink (a) the baserubber component. As (c) the crosslinking initiator, an organic peroxideis suitable. Specific examples of the organic peroxide include dicumylperoxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. Theseorganic peroxides may be used solely or as a mixture of at least two ofthem. Among them, dicumyl peroxide is preferably used.

The amount of (c) the crosslinking initiator is preferably 0.2 part bymass or more, more preferably 0.5 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.5 parts by massor less, with respect to 100 parts by mass of (a) the base rubber. Ifthe amount of (c) the crosslinking initiator is less than 0.2 part bymass, the constituent member formed from the rubber composition is sosoft that the resilience of the golf ball tends to be lowered. Inaddition, if the amount of (c) the crosslinking initiator exceeds 5.0parts by mass, the amount of (b) the co-crosslinking agent describedabove must be decreased such that the constituent member formed from therubber composition has an appropriate hardness, which tends to lower theresilience or worsen the durability of the golf ball.

((d) Metal Compound)

In the case that the rubber composition contains only theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as theco-crosslinking agent, the rubber composition preferably furthercontains (d) a metal compound. (d) The metal compound is notparticularly limited, as long as (d) the metal compound can neutralize(b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms inthe rubber composition. Examples of (d) the metal compound include: ametal hydroxide such as magnesium hydroxide, zinc hydroxide, calciumhydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, andcopper hydroxide; a metal oxide such as magnesium oxide, calcium oxide,zinc oxide, and copper oxide; and a metal carbonate such as magnesiumcarbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithiumcarbonate, and potassium carbonate. As (d) the metal compound, adivalent metal compound is preferable, a zinc compound is morepreferable. This is because the divalent metal compound reacts with theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and forms ametal crosslinking. In addition, if the zinc compound is used, the golfball having better resilience is obtained. (d) The metal compound may beused solely or as a mixture of at least two of them.

((e) Carboxylic Acid and/or Salt Thereof)

The rubber composition may further contain (e) a carboxylic acid and/ora salt thereof. If (e) the carboxylic acid and/or the salt thereof iscontained, the obtained spherical core has a greater outer-hard andinner-soft degree. Examples of (e) the carboxylic acid and/or the saltthereof include an aliphatic carboxylic acid, a salt of an aliphaticcarboxylic acid, an aromatic carboxylic acid, and a salt of an aromaticcarboxylic acid. (e) The carboxylic acid and/or the salt thereof may beused solely or as a mixture of at least two of them. As (e) thecarboxylic acid and/or the salt thereof, a carboxylic acid having 1 to30 carbon atoms and/or a salt thereof is preferable, a carboxylic acidhaving 4 to 30 carbon atoms and/or a salt thereof is more referable, acarboxylic acid having 5 to 25 carbon atoms and/or a salt thereof iseven more preferable. It should be noted that (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofused as the co-crosslinking agent is excluded from (e) the carboxylicacid/or the salt thereof.

Examples of (e) the aliphatic carboxylic acid and/or the salt thereofinclude a saturated fatty acid and/or a salt thereof, and an unsaturatedfatty acid and/or a salt thereof. Preferable examples of the saturatedfatty acid and/or the salt thereof include caprylic acid (octanoicacid), pelargonic acid (nonanoic acid), capric acid (decanoic acid),lauric acid, myristic acid, palmitic acid, stearic acid, behenic acidand oleic acid, and potassium salt, magnesium salt, calcium salt,aluminum salt, zinc salt, iron salt, copper salt, nickel salt, cobaltsalt thereof. Preferable examples of the unsaturated fatty acid and/orthe salt thereof include palmitoleic acid, oleic acid, linoleic acid,arachidonic acid, and potassium salt, magnesium salt, calcium salt,aluminum salt, zinc salt, iron salt, copper salt, nickel salt, cobaltsalt thereof.

Preferable examples of (e) the aromatic carboxylic acid and/or the saltthereof include benzoic acid, butylbenzoic acid, anisic acid(methoxybenzoic acid), dimethoxybenzoic acid, trimethoxybenzoic acid,dimethylaminobenzoic acid, chlorobenzoic acid, dichlorobenzoic acid,trichlorobenzoic acid, acetoxybenzoic acid, biphenylcarboxylic acid,naphthalenecarboxylic acid, anthracenecarboxylic acid, furancarboxylicacid, thenoic acid, and potassium salt, magnesium salt, calcium salt,aluminum salt, zinc salt, iron salt, copper salt, nickel salt, cobaltsalt thereof.

The amount of (e) the carboxylic acid and/or the salt thereof ispreferably 0.5 part by mass or more, more preferably 1.0 part by mass ormore, even more preferably 1.5 parts by mass or more, and is preferably40 parts by mass or less, more preferably 35 parts by mass or less, evenmore preferably 30 parts by mass or less, with respect to 100 parts bymass of (a) the base rubber. If the amount of (e) the carboxylic acidand/or the salt thereof is 0.5 part by mass or more, the spherical corehas a greater outer-hard and inner-soft degree, and if the amount of (e)the carboxylic acid and/or the salt thereof is 40 parts by mass or less,lowering in the core hardness is suppressed, and thus the resilience ofthe golf ball is better.

When a carboxylic acid having 1 to 14 carbon atoms and/or a salt thereofis used as (e) the carboxylic acid and/or the salt thereof, the amountof (e) the carboxylic acid and/or the salt thereof is preferably 1.0part by mass or more, more preferably 1.2 parts by mass or more, evenmore preferably 1.4 parts by mass or more, and is preferably 20 parts bymass or less, more preferably 18 parts by mass or less, even morepreferably 16 parts by mass or less, with respect to 100 parts by massof (a) the base rubber. When a carboxylic acid having 15 to 30 carbonatoms and/or a salt thereof is used as (e) the carboxylic acid and/orthe salt thereof, the amount of (e) the carboxylic acid and/or the saltthereof is preferably 5 parts by mass or more, more preferably 6 partsby mass or more, even more preferably 7 parts by mass or more, and ispreferably 40 parts by mass or less, more preferably 35 parts by mass orless, even more preferably 30 parts by mass or less, with respect to 100parts by mass of (a) the base rubber.

There are cases where the surface of the compound used as theco-crosslinking agent is treated with zinc stearate or the like toimprove the dispersibility to the rubber. It should be noted that, inthe case of using the co-crosslinking agent whose surface is treatedwith zinc stearate or the like, the amount of zinc stearate or the likeused as a surface treating agent is included in the amount of (e) thecarboxylic acid and/or the salt thereof. For example, if 25 parts bymass of zinc acrylate whose surface treatment amount with zinc stearateis 10 mass % is used, the amount of zinc stearate is 2.5 parts by massand the amount of zinc acrylate is 22.5 parts by mass. Thus, the amountof 2.5 parts by mass is counted as the amount of (e) the carboxylic acidand/or the salt thereof.

(f) Organic Sulfur Compound

The rubber composition preferably further contains (f) an organic sulfurcompound. If (f) the organic sulfur compound is contained, the obtainedspherical core has better resilience. Examples of (f) the organic sulfurcompound include thiophenols, thionaphthols, polysulfides, thiurams,thiocarboxylic acids, dithiocarboxylic acids, sulfenamides,dithiocarbamates, and thiazoles. From the viewpoint of increasing thehardness distribution of the spherical core, (f) the organic sulfurcompound is preferably an organic sulfur compound having a thiol group(—SH) or a metal salt thereof, and more preferably thiophenols,thionaphthols or metal salts thereof. (f) The organic sulfur compoundmay be used solely or as a mixture of at least two of them.

As (f) the organic sulfur compound, thiophenols and/or metal saltsthereof, thionaphthols and/or metal salts thereof, diphenyldisulfides,and thiuramdisulfides are preferable, and 2,4-dichlorothiophenol,2,6-difluorothiophenol, 2,6-dichlorothiophenol, 2,6-dibromothiophenol,2,6-diiodothiophenol, 2,4,5-trichlorothiophenol, pentachlorothiophenol,pentabromothiophenol, 1-thionaphthol, 2-thionaphthol, diphenyldisulfide,bis(2,6-difluorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide,bis(2,6-dibromophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, andbis(pentabromophenyl)disulfide are more preferable.

The amount of (f) the organic sulfur compound is preferably 0.05 part bymass or more, more preferably 0.1 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.0 parts by massor less, with respect to 100 parts by mass of (a) the base rubber. Ifthe amount of (f) the organic sulfur compound is 0.05 part by mass ormore, the obtained golf ball has better resilience, and if the amount of(f) the organic sulfur compound is 5.0 parts by mass or less, thecompression deformation amount of the obtained golf ball does not becomeexcessively large, and thus lowering in the resilience is suppressed.

The rubber composition may further contain an additive such as apigment, a filler for adjusting weight or the like, an antioxidant, apeptizing agent, and a softener, where necessary.

Examples of the pigment blended in the rubber composition include awhite pigment, a blue pigment, and a purple pigment. As the whitepigment, titanium oxide is preferably used. The type of titanium oxideis not particularly limited, but rutile type is preferably used becauseof the high opacity. In addition, the amount of titanium oxide ispreferably 0.5 part by mass or more, more preferably 2 parts by mass ormore, and is preferably 8 parts by mass or less, more preferably 5 partsby mass or less, with respect to 100 parts by mass of (a) the baserubber.

It is also preferred that the rubber composition contains both a whitepigment and a blue pigment. The blue pigment is blended in order tocause white color to be vivid, and examples thereof include ultramarineblue, cobalt blue, and phthalocyanine blue. In addition, examples of thepurple pigment include anthraquinone violet, dioxazine violet, andmethyl violet.

The amount of the blue pigment is preferably 0.001 part by mass or more,more preferably 0.05 part by mass or more, and is preferably 0.2 part bymass or less, more preferably 0.1 part by mass or less, with respect to100 parts by mass of (a) the base rubber. If the amount of the bluepigment is less than 0.001 part by mass, blueness is insufficient, andthe color looks yellowish. If the amount of the blue pigment exceeds 0.2part by mass, blueness is excessively strong, and a vivid whiteappearance is not provided.

The filler blended in the rubber composition is mainly used as a weightadjusting agent for adjusting the weight of the golf ball obtained as afinal product, and may be blended where necessary. Examples of thefiller include an inorganic filler such as zinc oxide, barium sulfate,calcium carbonate, magnesium oxide, tungsten powder, and molybdenumpowder. In particular, the filler is preferably zinc oxide. This isbecause it is thought that zinc oxide functions as a vulcanizationaccelerator to enhance the hardness of the constituent member as awhole. The amount of the filler is preferably 0.5 part by mass or more,more preferably 1 part by mass or more, and is preferably 30 parts bymass or less, more preferably 25 parts by mass or less, even morepreferably 20 parts by mass or less, with respect to 100 parts by massof the base rubber. If the amount of the filler is less than 0.5 part bymass, it is difficult to adjust the weight, and if the amount of thefiller exceeds 30 parts by mass, the weight proportion of the rubbercomponent is decreased and thus the resilience tends to be lowered.

The amount of the antioxidant is preferably 0.1 part by mass or more and1 part by mass or less with respect to 100 parts by mass of (a) the baserubber. In addition, the amount of the peptizing agent is preferably 0.1part by mass or more and 5 parts by mass or less with respect to 100parts by mass of (a) the base rubber.

The rubber composition may be obtained by mixing and kneading (a) thebase rubber, (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof, (c) the crosslinkinginitiator, and where necessary, other additives. The kneading may beconducted, without any limitation, for example, using a conventionalkneading machine such as a kneading roll and a banbury mixer.

The rubber composition may contain a crosslinked rubber powder. Thecrosslinked rubber is a rubber where chain rubber molecules arecrosslinked to form a three dimensional net structure such that noplastic deformation occurs. The crosslinking of chain rubber moleculesmay be carried out by using a co-crosslinking agent, an organicperoxide, sulfur and the like. The crosslinked rubber powder may beprepared from the rubber composition, or obtained by pulverizing a golfball core or offcuts produced when preparing a core.

The hardness of the crosslinked rubber powder is preferably 15 or more,more preferably 18 or more, even more preferably 20 or more, and ispreferably 65 or less, more preferably 60 or less, even more preferably58 or less in Shore C hardness. The volume average particle size of thecrosslinked rubber powder is preferably 200 μm or more, more preferably300 μm or more, even more preferably 400 μm or more, and is preferably800 μm or less, more preferably 750 μm or less, even more preferably 700μm or less.

The amount of the crosslinked rubber powder is preferably 1 part by massor more, more preferably 2 parts by mass or more, even more preferably 3parts by mass or more, and is preferably 40 parts by mass or less, morepreferably 30 parts by mass or less, even more preferably 20 parts bymass or less, with respect to 100 parts by mass of (a) the base rubber.

The spherical core may be obtained by molding the kneaded rubbercomposition in a mold. The temperature for molding the kneaded rubbercomposition is not particularly limited, and for example, is preferably120° C. or more, more preferably 150° C. or more, even more preferably160° C. or more, and is preferably 170° C. or less. If the moldingtemperature exceeds 170° C., the surface hardness of the core tends tobe lowered. In addition, the molding pressure preferably ranges from 2.9MPa to 11.8 MPa, and the molding time preferably ranges from 10 minutesto 60 minutes.

[Cover Material]

The cover is preferably formed from a resin composition containing aresin component. Examples of the resin component contained in the coverresin composition include an ionomer resin, a thermoplastic polyurethaneelastomer having a trade name of “Elastollan (registered trademark)”available from BASF Japan Ltd., a thermoplastic polyamide elastomerhaving a trade name of “Pebax (registered trademark)” available fromArkema K. K., a thermoplastic polyester elastomer having a trade name of“Hytrel (registered trademark)” available from Du Pont-Toray Co., Ltd.,and a thermoplastic styrene elastomer having a trade name of “Rabalon(registered trademark)” available from Mitsubishi Chemical Corporation.

Examples of the ionomer resin include a product obtained by neutralizingat least a part of carboxyl groups in a binary copolymer composed of anolefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atomswith a metal ion; a product obtained by neutralizing at least a part ofcarboxyl groups in a ternary copolymer composed of an olefin, anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and anα,β-unsaturated carboxylic acid ester with a metal ion; and a mixturethereof. The olefin is preferably an olefin having 2 to 8 carbon atoms.Examples of the olefin include ethylene, propylene, butene, pentene,hexene, heptene and octene, and ethylene is particularly preferred.Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms include acrylic acid, methacrylic acid, fumaric acid, maleic acidand crotonic acid, and acrylic acid or methacrylic acid is particularlypreferred. In addition, examples of the α,β-unsaturated carboxylic acidester include methyl ester, ethyl ester, propyl ester, n-butyl ester,isobutyl ester of acrylic acid, methacrylic acid, fumaric acid andmaleic acid, and acrylic acid ester or methacrylic acid ester isparticularly preferred. Among them, as the ionomer resin, a metal ionneutralized product of ethylene-(meth)acrylic acid binary copolymer or ametal ion neutralized product of ethylene-(meth)acrylicacid-(meth)acrylic acid ester ternary copolymer is preferred.

Specific examples of the ionomer resin include “Himilan (registeredtrademark)” available from Mitsui-Du Pont Polychemicals Co., Ltd.,“Surlyn (registered trademark)” available from E.I. du Pont de Nemoursand Company, and “lotek (registered trademark)” available from DownMobilChemical Corporation.

The cover resin composition preferably contains a thermoplasticpolyurethane elastomer or an ionomer resin as the resin component. It ispreferred that when the ionomer resin is used, a thermoplastic styreneelastomer is also used in combination. The amount of the thermoplasticpolyurethane elastomer or ionomer resin in the resin component of thecover resin composition is preferably 50 mass % or more, more preferably60 mass % or more, even more preferably 70 mass % or more.

The cover resin composition may further contain a pigment component suchas a white pigment (titanium oxide) and a blue pigment, a weightadjusting agent, a dispersant, an antioxidant, an ultraviolet absorber,a light stabilizer, a fluorescent material or fluorescent brightener, aslong as they do not impair the performance of the golf ball.

[First Resin Composition]

The first resin composition preferably contains (A) a thermoplasticresin, (B) an amphoteric surfactant, and (C) a fatty acid.

((A) Thermoplastic Resin)

(A) The thermoplastic resin preferably includes (A1) a metal ionneutralized product of a binary copolymer composed of an olefin and anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms (hereinafter,sometimes referred to as “(A1) binary ionomer resin”); and/or (A2) ametal ion neutralized product of a ternary copolymer composed of anolefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,and an α,β-unsaturated carboxylic acid ester (hereinafter, sometimesreferred to as “(A2) ternary ionomer resin”). (A1) The binary ionomerresin and (A2) the ternary ionomer resin are ionomer resins in which thecarboxyl groups of the copolymers are neutralized with a metal ion.

The olefin is preferably an olefin having 2 to 8 carbon atoms. Examplesof the olefin include ethylene, propylene, butene, pentene, hexene,heptene and octene, and ethylene is preferred. Examples of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms includeacrylic acid, methacrylic acid, fumaric acid, maleic acid and crotonicacid, and acrylic acid or methacrylic acid is preferred.

As the α,β-unsaturated carboxylic acid ester, an alkyl ester of anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms ispreferable, an alkyl ester of acrylic acid, methacrylic acid, fumaricacid or maleic acid is more preferable, an alkyl ester of acrylic acidor methacrylic acid is even more preferable. Examples of the alkyl groupconstituting the ester include methyl, ethyl, propyl, n-butyl, andisobutyl.

As (A1) the binary ionomer resin, a metal ion neutralized product ofethylene-(meth)acrylic acid binary copolymer is preferred. As (A2) theternary ionomer resin, a metal ion neutralized product ofethylene-(meth)acrylic acid-(meth)acrylic acid ester ternary copolymeris preferred. Herein, (meth)acrylic acid means acrylic acid and/ormethacrylic acid.

The amount of the α,β-unsaturated carboxylic acid component having 3 to8 carbon atoms in the binary copolymer constituting (A1) the binaryionomer resin and the ternary copolymer constituting (A2) the ternaryionomer resin is preferably 4 mass % or more, more preferably 6 mass %or more, even more preferably 8 mass % or more, and is preferably 50mass % or less, more preferably 30 mass % or less, even more preferably20 mass % or less, most preferably 15 mass % or less. If the amount ofthe α,β-unsaturated carboxylic acid component having 3 to 8 carbon atomsis 4 mass % or more, the ionomer resin has better resilience, and if theamount of the α,β-unsaturated carboxylic acid component having 3 to 8carbon atoms is 30 mass % or less, the ionomer resin has betterflexibility.

Examples of the metal ion for neutralizing at least a part of thecarboxyl groups in (A1) the binary ionomer resin and/or (A2) the ternaryionomer resin include a monovalent metal ion such as sodium, potassiumand lithium; a divalent metal ion such as magnesium, calcium, zinc,barium and cadmium; a trivalent metal ion such as aluminum; and othermetal ion such as tin and zirconium. It is preferred that (A1) thebinary ionomer resin and (A2) the ternary ionomer resin are neutralizedwith at least one metal ion selected from the group consisting of Na⁺,Mg²⁺, Ca²⁺, and Zn²⁺.

The neutralization degree of (A1) the binary ionomer resin and (A2) theternary ionomer resin is preferably 15 mole % or more, more preferably20 mole % or more, even more preferably 50 mole % or more, and ispreferably 100 mole % or less, more preferably 85 mole % or less. If theneutralization degree is 15 mole % or more, the obtained golf ball hasbetter resilience and durability. On the other hand, if theneutralization degree is 100 mole % or less, the golf ball resincomposition has better fluidity (better moldability). It should be notedthat the neutralization degree of the ionomer resin may be calculated bythe following expression.

Neutralization degree of ionomer resin (mole %)=100×(mole number ofneutralized carboxyl groups in copolymer / mole number of all carboxylgroups in copolymer)

As (A1) the binary ionomer resin and (A2) the ternary ionomer resin, anionomer resin which has been neutralized beforehand may be used; or amixture obtained by mixing a binary copolymer composed of an olefin andan α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or aternary copolymer composed of an olefin, an α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acidester with (E) a metal compound which will be described later, may beused. In addition, each of (A1) the binary ionomer resin and (A2) theternary ionomer resin may be used solely or as a mixture of at least twoof them.

Examples of (A1) the binary ionomer resin include Himilan (registeredtrademark) 1555 (Na), 1557 (Zn), 1605 (Na), 1706 (Zn), 1707 (Na), AM7311(Mg), AM7329 (Zn) (available from Du Pont-Mitsui Polychemicals Co.,Ltd.); Surlyn (registered trademark) 8945 (Na), 9945 (Zn), 8140 (Na),8150 (Na), 9120 (Zn), 9150 (Zn), 6910 (Mg), 6120 (Mg), 7930 (Li), 7940(Li), AD8546 (Li) (available from E.I. du Pont de Nemours and Company);and lotek (registered trademark) 8000 (Na), 8030 (Na), 7010 (Zn), 7030(Zn) (available from ExxonMobil Chemical Corporation).

Examples of (A2) the ternary ionomer resin include Himilan AM7327 (Zn),1855 (Zn), 1856 (Na), AM7331 (Na) (available from Du Pont-MitsuiPolychemicals Co., Ltd.); Surlyn 6320 (Mg), 8120 (Na), 8320 (Na), 9320(Zn), 9320W (Zn), HPF1000 (Mg), HPF2000 (Mg) (available from E.I. duPont de Nemours and Company); and lotek 7510 (Zn), 7520 (Zn) (availablefrom ExxonMobil Chemical Corporation).

Examples of the binary copolymer include NUCREL (registered trademark)N1050H, N2050H, N1110H, N0200H (available from Du Pont-MitsuiPolychemicals Co., Ltd.); and Primacor (registered trademark) 5980I(available from The DOW Chemical Company). Examples of the ternarycopolymer include NUCREL AN4318, AN4319 (available from Du Pont-MitsuiPolychemicals Co., Ltd.); and Primacor AT310, AT320 (available from TheDOW Chemical Company). It should be noted that Na, Zn, Li, Mg and thelike described in the parentheses after the trade names indicate metaltypes of neutralizing metal ions of the ionomer resins.

(A) The thermoplastic resin may further contain other thermoplasticresin than (A1) the binary ionomer resin and (A2) the ternary ionomerresin. In this case, the total amount of (A1) the binary ionomer resinand/or (A2) the ternary ionomer resin in (A) the thermoplastic resin ispreferably 50 mass % or more, more preferably 60 mass % or more, evenmore preferably 70 mass % or more. It is also preferred that (A) thethermoplastic resin consists of (A1) the binary ionomer resin and/or(A2) the ternary ionomer resin.

Examples of the other thermoplastic resin include a thermoplastic olefincopolymer, a thermoplastic polyurethane, a thermoplastic polyamide, athermoplastic styrene resin, a thermoplastic polyester, a thermoplasticacrylic resin, a thermoplastic polyolefin, a thermoplastic polydiene,and a thermoplastic polyether.

((B) Amphoteric Surfactant)

It is considered that (B) the amphoteric surfactant is taken into theion association of (A1) the binary ionomer resin and/or (A2) the ternaryionomer resin, and finely disperses the ion association to inhibitcrystallization of ethylene chains or weakens constraining of mainchains by the ion association. With these actions, the mobility of themolecular chain of the resin composition increases, and thus theresilience of the resin composition increases while retaining theflexibility.

(B) The amphoteric surfactant is not particularly limited, as long as ithas a cationic part and an anionic part within the molecule, and has anaction of lowering surface tension when being dissolved in water.Examples of (B) the amphoteric surfactant include a betaine typeamphoteric surfactant such as an alkylbetaine type, amidobetaine type,imidazoliumbetaine type, alkylsulfobetain type, amidosulfobetain type,and the like; an amidoamino acid type amphoteric surfactant and analkylamino fatty acid salt; an alkylamine oxide; α β-alanine typeamphoteric surfactant and a glycine type amphoteric surfactant; asulfobetaine type amphoteric surfactant; and a phosphobetaine typeamphoteric surfactant. (B) The amphoteric surfactant may be used solely,or at least two of them may be used in combination.

Specific examples of the amphoteric surfactant includedimethyllaurylbetaine, oleyldimethylamino acetic acid betaine(oleylbetaine), dimethyloleylbetaine, dimethylstearylbetaine,stearyldihydroxymethylbetaine, stearyldihydroxyethylbetaine,lauryldihydroxymethylbetaine, lauryldihydroxyethylbetaine,myristyldihydroxymethylbetaine, behenyldihydroxymethylbetaine,palmityldihydroxyethylbetaine, oleyldihydroxymethylbetaine, coconut oilfatty acid amidopropylbetaine, lauric acid amidoalkylbetaine,2-alkyl-N-carboxyalkylimidazoliumbetaine, lauric acidamidoalkylhydroxysulfobetaine, coconut oil fatty acidamidodialkylhydroxyalkylsulfobetaine, N-alkyl-β-aminopropionic acidsalt, N-alkyl-β-iminodipropionic acid salt, alkyldiaminoalkylglycine,alkylpolyaminoalkylglycine, sodium salt of alkylamino fatty acid,N,N-dimethyloctylamine oxide, N,N-dimethyllaurylamine oxide,N,N-dimethylstearylamine oxide, and the like.

The amount of (B) the amphoteric surfactant to be blended is preferably1 part by mass or more, more preferably 10 parts by mass or more, evenmore preferably 30 parts by mass or more, and is preferably 90 parts bymass or less, more preferably 80 parts by mass or less, even morepreferably 70 parts by mass or less, with respect to 100 parts by massof a total amount of (A1) the binary ionomer resin and (A2) the ternaryionomer resin. If the amount of (B) the amphoteric surfactant to beblended falls within the above range, the surfactant molecule is easilytaken into the ion association of the ionomer resin, thus the mobilityof the molecular chain of the ionomer resin increases, and theresilience of the golf ball resin composition increases while retainingthe flexibility.

((C) Fatty Acid)

If (C) a fatty acid is contained, the resin composition has betterfluidity, and thus it is easy to form a thin layer. (C) The fatty acidis not particularly limited, and a saturated fatty acid or anunsaturated fatty acid may be used. In addition, (C) the fatty acid maybe a linear fatty acid or a branched fatty acid. (C) The fatty acid maybe used solely or as a mixture of at least two of them.

The number of the carbon atom of (C) the fatty acid is preferably 4 ormore, more preferably 12 or more, even more preferably 16 or more, andis preferably 30 or less, more preferably 28 or less, even morepreferably 26 or less.

Examples of the saturated fatty acid include butanoic acid, pentanoicacid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid,decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid,tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoicacid, octadecanoic acid, nonadecanoic acid, icosanoic acid, henicosanoicacid, docosanoic acid, tricosanoic acid, tetracosanoic acid,pentacosanoic acid, hexacosanoic acid, heptacosanoic acid, octacosanoicacid, nonacosanoic acid, and triacontanoic acid.

Examples of the unsaturated fatty acid include butenoic acid, pentenoicacid, hexenoic acid, heptenoic acid, octenoic acid, nonenoic acid,decenoic acid, undecenoic acid, dodecenoic acid, tridecenoic acid,tetradecenoic acid, pentadecenoic acid, hexadecenoic acid, heptadecenoicacid, octadecenoic acid, nonadecenoic acid, icosenoic acid, henicosenoicacid, docosenoic acid, tricosenoic acid, tetracosenoic acid,pentacosenoic acid, hexacosenoic acid, heptacosenoic acid, octacosenoicacid, nonacosenoic acid, and triacontenoic acid.

The amount of (C) the fatty acid to be blended is preferably 10 parts bymass or more, more preferably 30 parts by mass or more, even morepreferably 60 parts by mass or more, and is preferably 150 parts by massor less, more preferably 120 parts by mass or less, even more preferably100 parts by mass or less, with respect to 100 parts by mass of (A) thethermoplastic resin. If the amount of (C) the fatty acid to be blendedis 10 parts by mass or more, the resin composition has better fluidity,and if the amount of (C) the fatty acid to be blended is 150 parts bymass or less, bleeding out of the fatty acid can be suppressed.

As (C) the fatty acid, a fatty acid salt may be used. Examples of thecation component of the fatty acid salt include a metal ion, an ammoniumion, and an organic cation. Examples of the metal ion include amonovalent metal ion such as sodium, potassium, lithium and silver; adivalent metal ion such as magnesium, calcium, zinc, barium, cadmium,copper, cobalt, nickel and manganese; a trivalent metal ion such asaluminum and iron; and other metal ion such as tin, zirconium andtitanium. The cation component may be used solely or as a mixture of atleast two of them.

((D) Metal Compound)

The first resin composition may further contain (D) a metal compound. If(D) the metal compound is contained, the neutralization degree of (A1)the binary ionomer resin and (A2) the ternary ionomer resin is furtherincreased, and thus the resin composition has better resilience.

(D) The metal compound is not particularly limited, as long as it canneutralize the carboxyl groups, and examples thereof include a metalhydroxide such as magnesium hydroxide, zinc hydroxide, calciumhydroxide, sodium hydroxide, hydroxide lithium, potassium hydroxide andcopper hydroxide; a metal oxide such as magnesium oxide, calcium oxide,zinc oxide and copper oxide; and a metal carbonate such as magnesiumcarbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithiumcarbonate and potassium carbonate.

The amount of (D) the metal compound to be blended may be appropriatelyadjusted according to the neutralization degree of (A1) the binaryionomer resin or (A2) the ternary ionomer resin, or the totalneutralization degree of the resin composition which will be describedlater.

The total neutralization degree of the first resin composition ispreferably more than 80 mole %, more preferably 85 mole % or more, evenmore preferably 90 mole % or more, and is preferably 160 mole % or less,more preferably 150 mole % or less, even more preferably 140 mole % orless. If the total neutralization degree is more than 80 mole %, theamount of the ion association increases, and thus the resin compositionhas better resilience. In addition, if the total neutralization degreeis 160 mole % or less, the resin composition has better fluidity. Itshould be noted that the total neutralization degree of the resincomposition is defined by the following formula.

Total neutralization degree (mole %)=100×[Σ(mole number of cationcomponent×valence of cation component)]/[Σ(mole number of anioncomponent×valence of anion component)]  [Formula 2]

In the formula, Σ(mole number of cation component×valence of cationcomponent) is a sum of a product obtained by multiplying the mole numberof the cation component by the valence of the cation component in thecomponent (A), a product obtained by multiplying the mole number of thecation component by the valence of the cation component in the component(B), and a product obtained by multiplying the mole number of the cationcomponent by the valence of the cation component in the component (C).It should be noted that, when the resin composition further contains thecomponent (D), Σ(mole number of cation component×valence of cationcomponent) further includes a product obtained by multiplying the molenumber of the cation-forming group or cation component by the valence ofthe cation-forming group or cation component in the component (D).

In the formula, Σ(mole number of anion component×valence of anioncomponent) is a sum of the mole number of the carboxyl group in thecomponent (A), the mole number of the carboxyl group in the component(B), and the mole number of the carboxyl group in the component (C).

It should be noted that, in the above formula, the cation component,cation-forming group, carboxyl group and anion-forming group include aunionized precursor. The amount of the cation component, the amount ofthe cation-forming group and the amount of the anion-forming group maybe determined, for example, by a neutralization titration method.

For example, when the resin composition contains the component (A),component (B), component (C) and component (D), Σ(mole number of cationcomponent×valence of cation component) is a sum of a product obtained bymultiplying the mole number of the metal ion by the valence of the metalion in the component (A), component (B), component (C) and component(D), and a product obtained by multiplying the mole number of thecation-forming group by the valence of the cation-forming group in thecomponent (B), and Σ(mole number of anion component×valence of anioncomponent) is a sum of the mole number of all the carboxyl groups in thecomponent (A), component (B) and component (C).

((F) Filler)

The first resin composition may further contain (F) a filler. As (F) thefiller, a conventional filler may be used. Preferable examples of (F)the filler include (F1) an organically modified layered silicate, (F2) acarbon nanotube, and (F3) a carbonaceous filler having a polarfunctional group on the surface thereof.

((F1) Organically Modified Layered Silicate)

A layered silicate is a silicate having a layered structure. Anorganically modified layered silicate is the one that is obtained byexchanging, with an organic onium ion, a part of or all the metalcations originally included between crystal layers in a layeredsilicate.

The layered silicate is not particularly limited as long as it is asilicate having a layered structure, and examples thereof include:layered silicates of kaolinites such as kaolinite, dickite, halloysite,chrysotile, lizardite, and amesite, layered silicates of smectites suchas montmorillonite, beidellite, nontronite, saponite, ferrous saponite,hectorite, sauconite, and stevensite, layered silicates of vermiculitessuch as dioctahedral vermiculite and trioctahedral vermiculite; layeredsilicates of micas such as white mica, paragonite, phlogopite, biotite,and lepidolite, layered silicates of brittle micas such as margarite,clintonite, and anandite, and layered silicates of chlorites such ascookeite, sudoite, clinochlore, chamosite, and nimite. These layeredsilicates may be natural or synthetic, and may be used solely or as amixture of two or more types. Among them, preferable examples of thelayered silicate include: layered silicates of smectites such asmontmorillonite, beidellite, nontronite, saponite, ferrous saponite,hectorite, sauconite, and stevensite, layered silicates of vermiculitessuch as dioctahedral vermiculite and trioctahedral vermiculite; andlayered silicates of micas such as white mica, paragonite, phlogopite,biotite, and lepidolite. Montmorillonite and layered silicates of micasare particularly suitable.

Each layer (primary particle) constituting the layered silicate ispreferably a nano size fine particle having a thickness of 10 nm orless, and preferably has a plate-like shape whose length and width are 1μm or less, respectively.

The amount of the organic substance in the organically modified layeredsilicate is preferably 10 mass % or more, more preferably 15 mass % ormore, even more preferably 25 mass % or more, and is preferably 70 mass% or less, more preferably 60 mass % or less, even more preferably 50mass % or less. If the amount of the organic substance is 10 mass % ormore, the organically modified layered silicate has betterdispersibility in the resin composition, and if the amount of theorganic substance is 70 mass % or less, the durability improvementeffect and resilience improvement effect due to the use of theorganically modified layered silicate are further enhanced. The amountof the organic substance is an ignition loss at 1000° C.

The organic onium ion used for organically modifying the layeredsilicate is a cation having a carbon chain. The organic onium ion is notparticularly limited, and examples thereof include organic ammoniumions, organic phosphonium ions, and organic sulfonium ions.

As the organic ammonium ion, any one of primary ammonium ion, secondaryammonium ion, tertiary ammonium ion, and quaternary ammonium ion may beused. Examples of the primary ammonium ion include decyl ammonium ion,dodecyl ammonium ion, octadecyl ammonium ion, ( )eyl ammonium ion, andbenzyl ammonium ion. Examples of the secondary ammonium ion includemethyl dodecyl ammonium ion and methyl octadecyl ammonium ion. Examplesof the tertiary ammonium ion include dimethyl dodecyl ammonium ion anddimethyl octadecyl ammonium ion.

Examples of the quaternary ammonium ion include: benzyl trialkylammonium ions such as benzyl trimethyl ammonium ion, benzyl triethylammonium ion, benzyl tributyl ammonium ion, benzyl dimethyl dodecylammonium ion, and benzyl dimethyl octadecyl ammonium ion; alkyltrimethyl ammonium ions such as trioctyl methyl ammonium ion, trimethyloctyl ammonium ion, trimethyl dodecyl ammonium ion, and trimethyloctadecyl ammonium ion; dimethyl dialkyl ammonium ions such as dimethyldioctyl ammonium ion, dimethyl didodecyl ammonium ion, and dimethyldioctadecyl ammonium ion; and oleyl bis(2-hydroxyethyl) methyl ammoniumion.

Other than those described above, examples of the organic ammonium ionalso include ammonium ions such as aniline, p-phenylene diamine,α-naphthylamine, p-aminodimethyl aniline, benzidine, pyridine,piperidine, and 6-aminocaproic acid.

Among the ammonium ions described above, a quaternary ammonium ionhaving a total of 11 to 30 intramolecular carbon atoms is particularlysuitable from a standpoint of dispersibility of the layered silicate andformability of ionic bonds. Specific examples thereof include octadecylammonium ion, trioctyl methyl ammonium ion, trimethyl octadecyl ammoniumion, benzyl dimethyl dodecyl ammonium ion, benzyl dimethyl octadecylammonium ion, dimethyl dioctadecyl ammonium ion, and oleylbis(2-hydroxyethyl) methyl ammonium ion.

The organically modified layered silicate may be produced by causing areaction between an organic onium ion and a layered silicate havingexchangeable metal ions between layers thereof by a method known in theart. Specific examples of the production method include a method ofperforming an ion exchange reaction in a polar solvent such as water,methanol, and ethanol; and a method of causing a direct reaction betweena liquid or melted ammonium salt and a layered silicate.

((F2) Carbon Nanotube)

The average diameter of (F2) the carbon nanotube in the transversedirection is preferably 0.02 μm or more, more preferably 0.05 μm ormore, and is preferably 0.3 μm or less, more preferably 0.25 μm or less.If the average diameter of the carbon nanotube in the transversedirection falls within the above range, the golf ball having excellentresilience while maintaining soft shot feeling is easily obtained.

The average length of (F2) the carbon nanotube in the longitudinaldirection is preferably 3 μm or more, more preferably 4 μm or more, andis preferably 50 μm or less, more preferably 30 μm or less. If theaverage length of the carbon nanotube in the longitudinal directionfalls within the above range, the golf ball having excellent resiliencewhile maintaining soft shot feeling is easily obtained.

It should be noted that the average length and average diameter of thecarbon nanotube may be measured by an imaging method. Specifically, theaverage length and average diameter are number based median average(d50) sizes in a certain direction of the particles, which is obtainedby analyzing the microscope photograph of the particles with an imageanalysis software (for example, Viewtrac (registered trademark)available from Nikkiso Co., Ltd.).

The average aspect ratio of (F2) the carbon nanotube is preferably 10 ormore, more preferably 15 or more, and is preferably 125 or less, morepreferably 50 or less. If the average aspect ratio of (F2) the carbonnanotube falls within the above range, the golf ball having excellentresilience while maintaining soft shot feeling is easily obtained. Itshould be noted that, in the present invention, the average aspect ratiomeans a ratio (average length in longitudinal direction/average diameterin transverse direction) of the average length of the filler in thelongitudinal direction to the average diameter of the filler in thetransverse direction.

((F3) Carbonaceous Filler Having a Polar Functional Group on the SurfaceThereof)

Examples of (F3) the carbonaceous filler having a polar functional groupon the surface thereof include a carbonaceous filler having a polarfunctional group directly bonding to the surface thereof, and acarbonaceous filler having the surface thereof coated with a polymerhaving a polar functional group. Examples of the material of (F3) thecarbonaceous filler include a natural graphite, synthetic graphite,carbon fiber, and carbon black. Among them, graphite is preferred,graphene and a graphite flake are particularly preferred. It should benoted that graphene is a sheet composed of a single layer having oneatom thickness peeled from graphite.

Examples of the polar functional group include a carboxyl group (—COON),hydroxy group (—OH), amino group (—NH₂), thiol group (—SH), sulfo group(—SO₃H), and phosphonic acid group (—PO (OH)₂).

The average diameter of (F3) the carbonaceous filler in the transversedirection is preferably 0.1 μm or more, more preferably 0.5 μm or more,and is preferably 100 μm or less. If the average diameter of (F3) thecarbonaceous filler in the transverse direction is 0.1 μm or more, thegolf ball resin composition has further enhanced bending stiffness, andif the average diameter of (F3) the carbonaceous filler in thetransverse direction is 100 μm or less, the golf ball resin compositionhas better fluidity and better flexibility.

The average diameter of (F3) the carbonaceous filler in the longitudinaldirection is preferably 0.2 μm or more, more preferably 3.0 μm or more,and is preferably 300 μm or less. If the average diameter of (F3) thecarbonaceous filler in the longitudinal direction is 0.2 μm or more, thegolf ball resin composition has further enhanced bending stiffness, andif the average diameter of (F3) the carbonaceous filler in thelongitudinal direction is 300 μm or less, the golf ball resincomposition has better fluidity and better flexibility.

The average aspect ratio of (F3) the carbonaceous filler is preferably2.0 or more, more preferably 5.0 or more, and is preferably 600 or less.If the average aspect ratio of (F3) the carbonaceous filler is 2.0 ormore, the golf ball resin composition has further enhanced bendingstiffness, and if the average aspect ratio of (F3) the carbonaceousfiller is 600 or less, the golf ball resin composition has betterfluidity and better flexibility.

The average thickness of (F3) the carbonaceous filler is preferably 1.0nm or more and 30 nm or less. If the average thickness of (F3) thecarbonaceous filler is 1.0 nm or more, the golf ball resin compositionhas further enhanced bending stiffness, and if the average thickness of(F3) the carbonaceous filler is 30 nm or less, the golf ball resincomposition has better flexibility.

The average diameter in the transverse direction, average diameter inthe longitudinal direction and thickness are decided by the side lengthsof the circumscribed cube of the particle. In other words, for thecircumscribed cube of the particle, the long axis thereof having thelongest axis is regarded as the long diameter (length), the short axisthereof having the shortest axis is regarded as the thickness (height),and the width thereof is regarded as the short diameter (width). Thenumber based average diameter in the transverse direction, averagediameter in the longitudinal direction and thickness of (F3) thecarbonaceous filler are obtained by measuring the short diameter, longdiameter and thickness of at least 50 particles with a scanning electronmicroscope (XL30ESEM available from Philips company), and calculatingthe average value thereof.

The amount of (F) the filler in the resin composition is preferably 5parts by mass or more, more preferably 8 parts by mass or more, evenmore preferably 10 parts by mass or more, and is preferably 60 parts bymass or less, more preferably 50 parts by mass or less, even morepreferably 40 parts by mass or less, with respect to 100 parts by massof the component (A). If the amount of (F) the filler falls within theabove range, the physical property improvement effect due to theaddition of (F) the filler is better, and lowering in toughness issuppressed. If (F) the filler is contained in the resin composition, theflexural modulus of the resin composition is enhanced and the spin rateof the golf ball on driver shots is lowered, and thus the golf balltravels a greater flight distance on driver shots.

Examples of the method of molding a cover resin composition into a coverare not particularly limited, and include a method which comprisesinjection molding the cover composition directly onto the core; and amethod which comprises molding the cover composition into a hollowshell, covering the core with a plurality of the hollow shells andperforming compression molding (preferably a method which comprisesmolding the cover composition into a hollow half-shell, covering thecore with two of the half-shells and performing compression molding).The golf ball body having the cover formed thereon is ejected from themold, and is preferably subjected to surface treatments such asdeburring, cleaning and sandblast where necessary. In addition, ifdesired, a mark may be formed.

The total number of dimples formed on the cover is preferably 200 ormore and 500 or less. If the total number of dimples is less than 200,the dimple effect is hardly obtained. On the other hand, if the totalnumber of dimples exceeds 500, the dimple effect is hardly obtainedbecause the size of the respective dimple is small. The shape (shape ina plan view) of the dimples formed on the cover includes, withoutlimitation, a circle; a polygonal shape such as a roughly triangularshape, a roughly quadrangular shape, a roughly pentagonal shape and aroughly hexagonal shape; and other irregular shape. These shapes may beemployed solely, or at least two of them may be employed in combination.

The golf ball body having the cover formed thereon is ejected from themold, and is preferably subjected to surface treatments such asdeburring, cleaning and sandblast where necessary. In addition, ifdesired, a paint film or a mark may be formed. The thickness of thepaint film is not particularly limited, and is preferably 5 μm or more,more preferably 7 μm or more, and is preferably 50 μm or less, morepreferably 40 μm or less, even more preferably 30 μm or less. If thethickness of the paint film is less than 5 μm, the paint film is easy towear off due to the continued use of the golf ball, and if the thicknessof the paint film exceeds 50 μm, the dimple effect is reduced and thusthe flight performance of the golf ball may be lowered.

EXAMPLES

Next, the present invention will be described in detail by way ofexamples. However, the present invention is not limited to the examplesdescribed below. Various changes and modifications without departingfrom the spirit of the present invention are included in the scope ofthe present invention.

[Evaluation Method] (1) Measurement of Particle Size

The dry powder sample was set into the dry-type unit of a laserdiffraction particle size analyzer (type: LMS-2000e, available fromSeishin Enterprise Co., Ltd.), the refractive index of the sample wasset as 1.52, and the particle size of the sample was measured. From theobtained volume based frequency distribution graph (the frequencydistribution graph obtained by dividing the particle size from 0.02 μmto 2000 μm in the logarithmic plot into 100 parts), the mode particlesize was obtained. In addition, from the obtained volume basedcumulative distribution graph, d10, the volume ratio (V₆₋₃₀₀) of theparticles having a particle size ranging from 6 μm to 300 μm, the volumeratio (V₀₋₂₀₀) of the particles having a particle size of 200 μm orless, and the specific surface area were obtained, respectively. Itshould be noted that, the measured value proximating to the particlesize of 300 μm or more was adopted as the cumulative ratio V % (300 μm),the measured value proximating to the particle size of 6 μm or less wasadopted as the cumulative ratio V % (6 μm), and the measured valueproximating to the particle size of 200 μm or less was adopted as thevolume ratio % of the particles having a particle size of 200 μm orless. The specific surface area was calculated from the particle size ofeach particle which was assumed to have a spherical shape.

(2) Hardness (Shore C) of Crosslinked Rubber Powder

At least three of rubber sheets (thickness: 2 mm) used for producing therubber powder were stacked on one another, and the hardness of the stackwas measured with an automatic hardness tester (Digitest II, availablefrom Bareiss company) using a testing device of “Shore C”.

(3) Core Hardness (Shore C)

The hardness measured at the surface of the core was adopted as thesurface hardness of the core. In addition, the core was cut into twohemispheres to obtain a cut plane, and the hardness at the central pointthereof and the hardness at predetermined distances from the centralpoint thereof were measured. It should be noted that the hardness wasmeasured at four points at predetermined distances from the centralpoint of the cut plane, and the average value thereof was adopted as thehardness of the core at the predetermined distance. The hardness wasmeasured with an automatic hardness tester (Digitest II, commerciallyavailable from Bareiss company) using a testing device of “Shore C”.

(4) Compression Deformation Amount (mm)

The deformation amount along the compression direction of the core(shrinking amount along the compression direction of the core), whenapplying a load from an initial load of 98 N to a final load of 1275 Nto the core, was measured.

(5) Slab Hardness (Shore D)

Sheets with a thickness of about 2 mm were produced by injection moldingthe cover composition. The sheets were stored at 23° C. for two weeks.At least three of these sheets were stacked on one another so as not tobe affected by the measuring substrate on which the sheets were placed,and the hardness of the stack was measured with an automatic hardnesstester (Digitest II, available from Bareiss company) using a testingdevice of “Shore D”.

(6) Bending Stiffness (kgf/cm²)

Test pieces with a thickness of about 2 mm, a width of 20 mm and alength of 100 mm were produced by heat press molding the golf ball resincomposition at 190° C. for 10 minutes. The test pieces were stored at atemperature of 23° C. plus or minus 2° C. and a relative humidity of 50%plus or minus 5% for 14 days. Load scales of the obtained test piece atpredetermined bending angles were measured with Olsen stiffness tester(available from Toyo Seiki Seisaku-sho, Ltd.), the bending angles (°)were plotted in the horizontal axis and the load scale readings wereplotted in the vertical axis to obtain a linear approximation curve, andthe slope of the linear approximation curve was calculated. Measurementwas carried out at a temperature of 23° C. plus or minus 2° C., relativehumidity of 50% plus or minus 5%, bending speed of 60°/min, and distancebetween fulcrums of 50 mm. The bending stiffness was calculated bymultiplying the slope value obtained above by 8.7078 and then dividingthe obtained product by the cube of thickness (cm) of the test piece. Itshould be noted that load scales at bending angles of 3°, 6°, 9° and 12°were measured to calculate the bending stiffness.

(7) Compression Test

The compression test for the golf ball was conducted with a precisionuniversal tester (AGX-100KN available from Shimadzu Corporation)provided with a thermostat chamber. The supporting plate was formed froma material of die steel (SKD11, quenched product), and had a diameter of100 mm and a thickness of 45.5 mm. The pressing plate was formed from amaterial of die steel (SKD11, quenched product), and had a diameter of100 mm and a thickness of 30 mm. In addition, the temperature inside thethermostat chamber, which was the measurement temperature, was set as−70° C.

The test was conducted as follows. First, the position of the pressingplate at the time of applying a force in a range from 5 N to 10 N withthe pressing plate was adopted as the initial position, and the heightof the golf ball at that time was measured. Next, the golf ball wascompressed with the pressing plate at a speed of 30 mm/min up to thecompression ratio of 7% (the position where the golf ball had a heightthat was 93% of the initial height) or the compression ratio of 19%(position where the golf ball had a height that was 81% of the initialheight), and then the pressing plate was returned to the initialposition at the same speed. Based on such a series of operations, aforce-deflection curve as shown in FIG. 12 was plotted.

The rebound equivalence energy ratio was calculated according to thefollowing expressions.

Rebound equivalence energy ratio (%)=100×(rebound equivalenceenergy/total applied energy)

Rebound equivalence energy=Area (dcbed)

Total applied energy=Area (oabeo)

Herein, Area (dcbed) is an area surrounded by dcbed of theforce-deflection curve, and Area (oabeo) is an area surrounded by oabeoof the force-deflection curve.

(8) Coefficient of Restitution

A metal cylindrical object with a mass of 198.4 g was allowed to collidewith the golf ball at a speed of 40 m/sec or 50 m/sec, and the speeds ofthe cylindrical object and the golf ball before and after the collisionwere measured. Based on these speeds and the mass of each object,coefficient of restitution of each golf ball was calculated. It shouldbe noted that the measurement was conducted using twelve samples foreach golf ball, and the average value thereof was adopted as thecoefficient of restitution of the golf ball. In Tables 5 to 7, thecoefficient of restitution of golf balls is shown as the difference fromthat of the golf ball No. 7.

(9) Spin Rate on Driver Shots (rpm)

A W#1 driver provided with a metal head (XXIO, Shaft: S, loft angel:11°, available from Dunlop Sports Limited) was installed on a swingrobot M/C available from Golf Laboratories, Inc. The golf ball was hitat a head speed of 40 m/sec, and the spin rate right after hitting thegolf ball was measured. This measurement was conducted twelve times foreach golf ball, and the average value thereof was adopted as themeasurement value for the golf ball. A sequence of photographs of thehit golf ball were taken for measuring the spin rate right after hittingthe golf ball. In Tables 5 to 7, the flight distance and the spin rateof the golf ball on driver shots are shown as the difference from thoseof the golf ball No. 7.

[Synthesis of Zinc Acrylate] ZDA-1

A suspension was prepared by adding 1140 g of a solvent and 5 moles ofzinc oxide into a jacketed kneader and agitating the obtained mixture.While keeping the temperature inside the kneader at 5° C. to 40° C., 10moles of acrylic acid was slowly added into the suspension for about 3hours to cause a reaction between zinc oxide and acrylic acid, and thenthe temperature inside the kneader was set as 40° C. After finishing theaddition of acrylic acid, the reaction was further continued for 4 hoursat 40° C. Then, while increasing the temperature of the reaction liquidto 50° C. slowly such that a reduced pressure of 20 Torr was obtained,water generated in the reaction and the solvent were removed bydistillation and dried for 5 hours, to obtain 5 moles of zinc acrylate.The above obtained zinc acrylate was air flow classified to obtain zincacrylate (ZDA-1). The zinc acrylate (ZDA-1) has a mode particle size of22.9 μm and includes particles having a particle size ranging from 6 μmto 300 μm in a volume ratio of about 70%.

The following apparatuses were used in the air flow classification.

Supplier: table feeder ZGJ-200

Classifier: CLASSIEL N-5 (available from Seishin Enterprise Co., Ltd.)

Collector: bag filter TD-270 (available from Seishin Enterprise Co.,Ltd.)

ZDA-2

The above obtained unclassified zinc acrylate was air flow classified toobtain zinc acrylate (ZDA-2). Zinc acrylate (ZDA-2) has a mode particlesize of 20.0 μm and a volume ratio of particles having a particle sizeranging from 6 μm to 300 μm of about 93%.

The following apparatuses were used in the air flow classification.

Supplier: table feeder ZGJ-200

Classifier: CLASSIEL N-5 (available from Seishin Enterprise Co., Ltd.)

Collector: bag filter TD-270 (available from Seishin Enterprise Co.,Ltd.)

ZDA-3

The unclassified zinc acrylate obtained in the synthesis of ZDA-1 wasair flow classified to obtain zinc acrylate (ZDA-3). The zinc acrylate(ZDA-3) has a mode particle size of 18.7 μm and a volume ratio ofparticles having a particle size ranging from 6 μm to 300 μm of about85%.

The following apparatuses were used in the air flow classification.

Supplier: table feeder ZGJ-200

Classifier: CLASSIEL N-01 (available from Seishin Enterprise Co., Ltd.)

Collector: bag filter TD-270 (available from Seishin Enterprise Co.,Ltd.)

The properties of ZDA-1 to ZDA-3 are summarized in Table 1. It should benoted that the properties of Sanceler SR and ZD-DA90S are also shown inTable 1 for reference. Sanceler SR is zinc acrylate coated with stearicacid, and ZD-DA90S is a mixture of zinc acrylate and zinc stearate.

TABLE 1 Mode Volume Volume Specific Material particle d10 ratio ratiosurface area particle size (μm) (μm) V₆₋₃₀₀ (%) V₀₋₂₀₀ (%) (m²/g) ZDA-122.9 7.0 70.0 76.6 0.43 ZDA-2 20.0 10.4 93.5 92.0 0.32 ZDA-3 18.7 10.585.5 81.5 0.29 Sanceler SR 4.5 2.0 51.8 97.9 1.75 ZN-DA90S 5.6 2.1 57.898.0 1.65

Zinc stearate was added into the obtained ZDA-1, ZDA-2 and ZDA-3,respectively, the resultant mixture was mixed to treat the particlesurface of zinc acrylate with zinc stearate (zinc stearate treatmentamount: 10 mass %).

[Preparation of Crosslinked Rubber Powder]

The rubber compositions having the formulations shown in Table 2 werekneaded with a kneading roll, and then heated at 170° C. for 20 minutesto obtain a rubber sheet (thickness: 2 mm). The obtained rubber sheetwas pulverized with a frozen pulverizer to obtain the crosslinked rubberpowders having a volume average particle size ranging from 400 μm to 700μm. It should be noted that the obtained rubber sheet has a uniformhardness.

TABLE 2 Crosslinked Crosslinked rubber powder A rubber powder BFormulation BR730 100 100 (parts by ZN-DA90S 28 15 mass) Zinc oxide 5 5Barium sulfate 10 10 Dicumyl peroxide 0.16 0.16 2-Thionaphthol 0.2 0.2Rubber powder 5 5 Hardness (Shore C) 57.3 39.7 BR730: high-cispolybutadiene (cis-1,4 bond amount = 96 mass %, 1,2-vinyl bond amount =1.3 mass %, Moony viscosity (ML₁₊₄ (100° C.)) = 55, molecular weightdistribution (Mw/Mn) = 3) available from JSR Corporation ZN-DA90S: zincacrylate (a mixture with 10 mass % of zinc stearate) available fromNippon Shokubai Co., Ltd. Zinc oxide: “Ginrei R” available from TohoZinc Co., Ltd. Dicumyl peroxide: “PERCUMYL (registered trademark) D”available from NOF Corporation 2-Thionaphthol: available from TokyoChemical Industry Co., Ltd. Rubber powder: a powder obtained bypulverizing a golf ball core formed from a rubber composition

[Production of Golf Ball] (1) Production of Core

The rubber compositions having the formulations shown in Table 3 werekneaded with a kneading roll, and then heat-pressed in upper and lowermolds, each having a hemispherical cavity, at 170° C. for 20 minutes toproduce the spherical cores having a diameter of 39.8 mm. It should benoted that the zinc acrylate particles originating from ZDA-1, ZDA-2 andZDA-3 are remained inside the molded spherical cores No. a, b and c.

TABLE 3 Core No. a b c d e f g h i j Rubber composition BR730 100 100100 100 100 100 100 100 100 100 (parts by mass) ZDA-1 (coated with 39 —— — — — — — — — zinc stearate) ZDA-2 (coated with — 39 — — — — — — — —zinc stearate) ZDA-3 (coated with — — 41 — — — — — — — zinc stearate)Sanceler SR — — — 42 30 30 — — 42 42 ZN-DA90S — — — — — — 30 30 — —2-Thionaphthol 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Zinc oxide 5 5 55 5 5 5 5 5 5 Zinc octanoate — — — 7.5 — — — — 7.5 7.5 Zinc stearate — —— — 10 — — 10 — — Dicumyl peroxide 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.80.8 Barium sulfate *1) *1) *1) *1) *1) *1) *1) *1) *1) *1) Crosslinkedrubber — — — — — — — — 5 — powder A Crosslinked rubber — — — — — — — — —5 powder B Core hardness Center hardness Ho 60.3 59.8 62.3 52.1 57.162.8 63.1 57.1 52.1 52.1 distribution 12.5% point hardness 65.9 65.267.2 56.7 64.0 69.5 69.0 64.0 56.7 56.7 (Shore C) 25.0% point hardness69.1 68.5 69.8 62.0 68.1 72.6 73.3 68.1 62.0 62.0 37.5% point hardness70.1 69.6 71.3 64.9 69.4 73.8 74.8 69.4 64.9 64.9 50.0% point hardness70.2 69.8 71.6 64.8 69.4 74.0 74.5 69.4 64.8 64.8 62.5% point hardness71.1 71.1 73.1 73.7 70.0 73.1 73.2 70.0 73.7 73.7 75.0% point hardness78.0 77.5 78.4 79.1 77.7 77.2 79.2 77.7 79.1 79.1 87.5% point hardness81.4 80.9 84.3 77.0 81.8 80.8 81.8 81.8 77.0 77.0 Surface hardness Hs87.9 87.3 91.2 87.1 86.7 86.8 87.8 86.7 87.1 87.1 Hardness difference27.7 27.6 28.9 35.0 29.6 24.1 24.8 29.6 35.0 35.0 (Hs − Ho) R² ofapproximation 0.91 0.92 0.91 0.96 0.92 0.87 0.88 0.92 0.96 0.96 curveSlope of 0.23 0.24 0.25 0.33 0.26 0.18 0.20 0.26 0.33 0.33 approximationcurve Core compression 3.3 3.3 3.3 3.3 3.3 3.1 3.1 3.3 3.3 3.3deformation amount (mm) *1) The amount was adjusted such that thefinally obtained golf ball had a mass of 40.0 g. BR730: high-cispolybutadiene (cis-1,4 bond amount = 96 mass %, 1,2-vinyl bond amount =1.3 mass %, Moony viscosity (ML₁₊₄ (100° C.)) = 55, molecular weightdistribution (Mw/Mn) = 3) available from JSR Corporation Sanceler SR:zinc acrylate (a product coated with 10 mass % of stearic acid)available from Sanshin Chemical Industry Co., Ltd. ZN-DA90S: zincacrylate (a mixture with 10 mass % of zinc stearate) available fromNippon Shokubai Co., Ltd. 2-Thionaphthol: available from Tokyo ChemicalIndustry Co., Ltd. Zinc oxide: “Ginrei R” available from Toho Zinc Co.,Ltd. Zinc octanoate: available from Mitsuwa Chemicals Co., Ltd. Zincstearate: available from Wako Pure Chemical Industries, Ltd. Dicumylperoxide: “PERCUMYL (registered trademark) D” available from NOFCorporation Barium sulfate: “Barium sulfate BD” available from SakaiChemical Industry Co., Ltd.

(2) Production of Cover

Next, according to the formulations shown in Table 4, the covermaterials were extruded with a twin-screw kneading extruder to preparethe cover compositions in a pellet form. The extruding conditions were ascrew diameter of 45 mm, a screw rotational speed of 200 rpm, and screwL/D=35, and the mixture was heated to 150 to 230° C. at the die positionof the extruder. The obtained cover composition was injection moldedonto the spherical core obtained above to form an inner cover, and thenthe cover composition was injection molded onto the inner cover to forman outermost cover.

TABLE 4 Resin composition No. A B C D E F G H I Formulation NUCREL N1560100 100 100 100 100 100 100 100 100 (parts by Oleic acid 90 90 90 90 9090 90 90 90 mass) Oleylbetaine 60 60 60 60 60 60 60 60 60 Magnesiumoxide 18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2 18.2 S-BEN NO12 — 10 20 3035 40 45 — — S-BEN NX — — — — — — — 10 — S-BEN NO12S — — — — — — — — 10S-BEN E — — — — — — — — — TPP-treated Kunipia — — — — — — — — — BEN-GELA — — — — — — — — — VGCF-H — — — — — — — — — Rap dGO — — — — — — — — —Himilan 1601 — — — — — — — — — Himilan 1557 — — — — — — — — — ElastollanNY82A10 — — — — — — — — — Titanium oxide — — — — — — — — — PropertiesSlab hardness 46 52 54 55 55 56 56 52 53 (Shore D) Bending stiffness 5102329 2724 3432 3782 4132 4482 2879 1332 (kgf/cm²) Slab hardness/ 0.090.02 0.02 0.02 0.01 0.01 0.01 0.02 0.04 bending stiffness Neutralization120 120 120 120 120 120 120 120 120 degree (%) Resin composition No. J KL M N O P Q R Formulation NUCREL N1560 100 100 100 100 100 100 100 — —(parts by Oleic acid 90 90 90 90 90 90 90 — — mass) Oleylbetaine 60 6060 60 60 60 60 — — Magnesium oxide 18.2 18.2 18.2 18.2 18.2 18.2 2.2 — —S-BEN NO12 — — — — — 2 10 — — S-BEN NX — — — — — — — — — S-BEN NO12S — —— — — — — — — S-BEN E 10 — — — — — — — — TPP-treated Kunipia — 10 — — —— — — — BEN-GEL A — — 10 — — — — — — VGCF-H — — — 20 — — — — — Rap dGO —— — — 20 — — — — Himilan 1601 — — — — — — — 50 — Himilan 1557 — — — — —— — 50 — Elastollan NY82A10 — — — — — — — — 100 Titanium oxide — — — — —— —  4  4 Properties Slab hardness 53 51 49 50 51 53 53 65  29 (Shore D)Bending stiffness 1532 1578 1185 1756 2639 817 1495 — — (kgf/cm²) Slabhardness/ 0.03 0.03 0.04 0.03 0.02 0.06 0.04 — — bending stiffnessNeutralization 120 120 120 120 120 120 80 — — degree (%) NUCREL(registered trademark) N1560: ethylene-methacrylic acid copolymer (acidcomponent amount: 15 mass %, melt flow rate (190° C., 2.16 kgf): 60 g/10min, Shore D hardness: 53) available from Du Pont-Mitsui PolychemicalsCo., Ltd. Oleic acid: available from Tokyo Chemical Industry Co., Ltd.Oleylbetaine: a purified product obtained by removing water and saltfrom “Chembetaine OL” available from The Lubrizol Corporation Magnesiumhydroxide: available from Wako Pure Chemical Industries, Ltd. S-BENNO12: quaternary ammonium-treated montmorillonite (quaternary ammoniumcation: a mixture of dimethyl distearyl ammonium ion and oleylbis(2-hydroxyethyl) methyl ammonium ion in a mass ratio of 1:1, organicsubstance amount: 38.8 mass %) available from Hojun Co., Ltd. S-BEN NX:quaternary ammonium-treated montmorillonite (quaternary ammonium cation:dimethyl distearyl ammonium ion, organic substance amount: 41.8 mass %)available from Hojun Co., Ltd. S-BEN NO12S: quaternary ammonium-treatedmontmorillonite (quaternary ammonium cation: oleyl bis(2-hydroxyethyl)methyl ammonium ion, organic substance amount: 31.5 mass %) availablefrom Hojun Co., Ltd. S-BEN E: quaternary ammonium-treatedmontmorillonite (quaternary ammonium cation: trimethyl stearyl ammoniumion, organic substance amount: 25.6 mass %) available from Hojun Co.,Ltd. TPP-treated Kunipia: organically treated montmorillonite (organiccation: tetraphenyl phosphonium ion) available from Kunimine IndustriesCo., Ltd. BEN-GEL A: montmorillonite available from Hojun Co., Ltd.VGCF-H: carbon nanotube (average diameter in transverse direction: 0.15μm, average length in longitudinal direction: 7 μm, average aspectratio: 46.7) available from Showa Denko K.K. Rap dGO: graphene oxide(average short diameter: 2 μm, average long diameter: 20 μm, averageaspect ratio: 10, average thickness: 5 nm, functional group type:carboxyl group and hydroxyl group, functional group amount: 1.2 mmol/g)available from Nishina Material Company Himilan (registered trademark)1601: sodium ion-neutralized ionomer resin available from Du Pont-MitsuiPolychemicals Co., Ltd. Himilan 1557: zinc ion-neutralized ionomer resinavailable from Du Pont-Mitsui Polychemicals Co., Ltd. Elastollan(registered trademark) NY82A10: thermoplastic polyurethane elastomeravailable from BASF Japan Ltd.

Evaluation results for each golf ball are shown in Tables 5 to 7.

TABLE 5 Golf ball No. 1 2 3 4 5 6 Core Core No. a b c i j d Diameter(mm) 39.7 39.7 39.7 39.7 39.7 39.7 Hardness difference (Hs − Ho) (ShoreC) 27.7 27.6 28.9 35.0 35.0 35.0 R² of approximation curve 0.91 0.920.91 0.96 0.96 0.96 Slope of approximation curve 0.23 0.24 0.25 0.330.33 0.33 Compression deformation amount (mm) 3.3 3.3 3.3 3.3 3.3 3.3Inner cover Resin composition No. A A A A A A Thickness (mm) 1.0 1.0 1.01.0 1.0 1.0 Slab hardness (Shore D) 46 46 46 46 46 46 Bending stiffness(kgf/cm²) 510 510 510 510 510 510 Neutralization degree (%) 120 120 120120 120 120 Outer cover Resin composition No. R R R R R R Thickness (mm)0.5 0.5 0.5 0.5 0.5 0.5 Properties Compression deformation amount (mm)2.8 2.8 2.8 2.8 2.8 2.8 At deformation Rebound equivalence 4.21 4.134.10 3.96 3.96 3.81 amount of 7% energy (N · m) Applied energy (N · m)6.31 6.21 6.18 6.01 6.01 5.82 Rebound equivalence 66.7 66.5 66.4 65.9265.92 65.46 energy ratio (%) At deformation Rebound equivalence 21.1021.10 21.11 21.10 21.10 21.10 amount of 19% energy (N · m) Appliedenergy (N · m) 35.40 35.40 35.40 35.40 35.40 35.40 Rebound equivalence59.6 59.6 59.6 59.6 59.6 59.6 energy ratio (%) Evaluation Coefficient ofrestitution (at 40 m/sec) 0.011 0.009 0.008 0.004 0.004 0 Coefficient ofrestitution (at 50 m/sec) 0 0.001 0.002 0 0 0 Spin rate on driver shots(rpm) −110 −100 −80 −100 −100 −125 Golf ball No. 7 8 9 10 Core Core No.f g h a Diameter (mm) 39.7 39.7 39.7 39.7 Hardness difference (Hs − Ho)(Shore C) 24.1 24.8 29.6 27.7 R² of approximation curve 0.87 0.88 0.920.91 Slope of approximation curve 0.18 0.20 0.26 0.23 Compressiondeformation amount (mm) 3.1 3.1 3.3 3.3 Inner cover Resin compositionNo. A A A Q Thickness (mm) 1.0 1.0 1.0 1.5 Slab hardness (Shore D) 46 4646 65 Bending stiffness (kgf/cm²) 510 510 510 — Neutralization degree(%) 120 120 120 — Outer cover Resin composition No. R R R — Thickness(mm) 0.5 0.5 0.5 — Properties Compression deformation amount (mm) 2.72.7 2.7 2.8 At deformation Rebound equivalence 3.81 3.81 3.81 3.81amount of 7% energy (N · m) Applied energy (N · m) 5.82 5.82 5.82 5.82Rebound equivalence 65.46 65.46 65.46 65.46 energy ratio (%) Atdeformation Rebound equivalence 21.10 21.10 21.10 21.10 amount of 19%energy (N · m) Applied energy (N · m) 35.40 35.40 35.40 35.40 Reboundequivalence 59.6 59.6 59.6 59.6 energy ratio (%) Evaluation Coefficientof restitution (at 40 m/sec) Standard 0 0 0 Coefficient of restitution(at 50 m/sec) Standard 0 0 0 Spin rate on driver shots (rpm) 0 5 −60−100

TABLE 6 Golf ball No. 11 12 13 14 15 16 17 Core Core No. e e e e e e eDiameter (mm) 39.7 39.7 39.7 39.7 39.7 39.7 39.7 Hardness difference (Hs− Ho) (Shore C) 29.6 29.6 29.6 29.6 29.6 29.6 29.6 R² of approximationcurve 0.92 0.92 0.92 0.92 0.92 0.92 0.92 Slope of approximation curve0.26 0.26 0.26 0.26 0.26 0.26 0.26 Compression deformation amount (mm)3.3 3.3 3.3 3.3 3.3 3.3 3.3 Inner cover Resin composition No. B C D F HI J Thickness (mm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Slab hardness (Shore D)52 54 55 56 52 53 53 Bending stiffness (kgf/cm²) 2329 2724 3432 41322879 1332 1532 Neutralization degree (%) 120 120 120 120 120 120 120Outer cover Resin composition No. R R R R R R R Thickness (mm) 0.5 0.50.5 0.5 0.5 0.5 0.5 Properties Compression deformation amount (mm) 2.72.7 2.7 2.7 2.7 2.7 2.7 At deformation Rebound equivalence 4.06 4.104.13 4.17 4.06 4.06 4.06 amount of 7% energy (N · m) Applied energy (N ·m) 6.13 6.18 6.21 6.26 6.13 6.13 6.13 Rebound equivalence 66.3 66.4 66.566.6 66.3 66.3 66.3 energy ratio (%) At deformation Rebound equivalence21.11 21.12 21.10 21.10 21.11 21.11 21.11 amount of 19% energy (N · m)Applied energy (N · m) 35.40 35.40 35.40 35.40 35.40 35.40 35.40 Reboundequivalence 59.6 59.6 59.6 59.6 59.6 59.6 59.6 energy ratio (%)Evaluation Coefficient of restitution (at 40 m/sec) 0.007 0.008 0.0090.010 0.007 0.007 0.007 Coefficient of restitution (at 50 m/sec) 0.0030.002 0.001 0 0.003 0.003 0.003 Spin rate on driver shots (rpm) −60 −60−60 −60 −60 −60 −60 Golf ball No. 18 19 20 21 22 23 Core Core No. e e ee e e Diameter (mm) 39.7 39.7 39.7 39.7 39.7 39.7 Hardness difference(Hs − Ho) (Shore C) 29.6 29.6 29.6 29.6 29.6 29.6 R² of approximationcurve 0.92 0.92 0.92 0.92 0.92 0.92 Slope of approximation curve 0.260.26 0.26 0.26 0.26 0.26 Compression deformation amount (mm) 3.3 3.3 3.33.3 3.3 3.3 Inner cover Resin composition No. K L M N O P Thickness (mm)1.0 1.0 1.0 1.0 1.0 1.0 Slab hardness (Shore D) 51 49 50 51 53 53Bending stiffness (kgf/cm²) 1578 1185 1756 2639 817 1495 Neutralizationdegree (%) 120 120 120 120 120 80 Outer cover Resin composition No. R RR R R R Thickness (mm) 0.5 0.5 0.5 0.5 0.5 0.5 Properties Compressiondeformation amount (mm) 2.7 2.7 2.8 2.8 2.8 2.8 At deformation Reboundequivalence 4.06 4.06 4.06 4.10 3.81 3.81 amount of 7% energy (N · m)Applied energy (N · m) 6.13 6.13 6.13 6.18 5.82 5.82 Rebound equivalence66.3 66.3 66.27 66.38 65.46 65.46 energy ratio (%) At deformationRebound equivalence 21.11 21.11 21.10 21.10 21.10 21.10 amount of 19%energy (N · m) Applied energy (N · m) 35.40 35.40 35.40 35.40 35.4035.40 Rebound equivalence 59.6 59.6 59.6 59.6 59.6 59.6 energy ratio (%)Evaluation Coefficient of restitution (at 40 m/sec) 0.007 0.007 0.0070.008 0 0 Coefficient of restitution (at 50 m/sec) 0.003 0.003 0 0 0 0Spin rate on driver shots (rpm) −60 −60 −60 −60 −60 −60

TABLE 7 Golf ball No. 24 25 26 27 28 29 30 31 Core Core No. d d d d a aa a Diameter (mm) 39.7 39.7 39.7 39.7 39.7 39.7 39.7 39.7 Hardnessdifference (Hs − Ho) (Shore C) 35.0 35.0 35.0 35.0 27.7 27.7 27.7 27.7R² of approximation curve 0.96 0.96 0.96 0.96 0.91 0.91 0.91 0.91 Slopeof approximation curve 0.33 0.33 0.33 0.33 0.23 0.23 0.23 0.23Compression deformation amount (mm) 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3Inner Resin composition No. C D E G C D E G cover Thickness (mm) 1.0 1.01.0 1.0 1.0 1.0 1.0 1.0 Slab hardness (Shore D) 54 55 55 56 54 55 55 56Bending stiffness (kgf/cm²) 2724 3432 3782 4482 2724 3432 3782 4482Neutralization degree (%) 120 120 120 120 120 120 120 120 Outer Resincomposition No. R R R R R R R R cover Thickness (mm) 0.5 0.5 0.5 0.5 0.50.5 0.5 0.5 Proper- Compression deformation amount (mm) 2.8 2.8 2.8 2.82.8 2.8 2.8 2.8 ties At deformation Rebound equivalence 4.10 4.13 4.174.21 4.17 4.25 4.31 4.39 amount of 7% energy (N · m) Applied energy (N ·m) 6.18 6.21 6.26 6.31 6.26 6.36 6.43 6.52 Rebound equivalence 66.4 66.566.6 66.7 66.61 66.84 67.07 67.30 energy ratio (%) At deformationRebound equivalence 21.11 21.10 21.10 21.10 21.10 21.10 21.10 21.10amount of 19% energy (N · m) Applied energy (N · m) 35.40 35.40 35.4035.40 35.40 35.40 35.40 35.40 Rebound equivalence 59.6 59.6 59.6 59.659.6 59.6 59.6 59.6 energy ratio (%) Evalu- Coefficient of restitution(at 40 m/sec) 0.008 0.009 0.010 0.011 0.010 0.012 0.014 0.016 ationCoefficient of restitution (at 50 m/sec) 0.002 0.001 0 0 0 0 0 0 Spinrate on driver shots (rpm) −125 −125 −125 −125 −100 −100 −100 −100

The golf balls No. 1 to 5, 11 to 21 and 24 to 25 have a reboundequivalence energy ratio (R₄₀) ranging from 65.50% to 99.0% and arebound equivalence energy ratio (R₅₀) ranging from 20.0% to 70.0%. Eachof these golf balls has a coefficient of restitution (at 50 m/sec) whichis equal to or larger than that of the golf ball No. 7, and has acoefficient of restitution (at 40 m/sec) which is larger than that ofthe golf ball No. 7. The golf balls No. 6, 8 to 10, 22 and 23 have arebound equivalence energy ratio (R₄₀) of less than 65.50%. These golfballs have a coefficient of restitution (at 40 m/sec) that is notimproved compared to the golf ball No. 7.

This application is based on Japanese patent application No. 2015-152346filed on Jul. 31, 2015, the contents of which are hereby incorporated byreference.

1. A golf ball comprising a spherical core and a cover covering thespherical core, wherein the golf ball has a rebound equivalence energyratio (R₄₀) ranging from 65.50% to 99.0% at a deformation amount of 7%,and a rebound equivalence energy ratio (R₅₀) ranging from 20.0% to 70.0%at a deformation amount of 19%, in a compression test (measurementtemperature: −70° C., compression speed: 30 mm/min) applying a load tothe golf ball along a radial direction of the golf ball.
 2. The golfball according to claim 1, wherein a difference (R₄₀-R₅₀) between therebound equivalence energy ratio (R₄₀) and the rebound equivalenceenergy ratio (R₅₀) is 1.0% or more.
 3. The golf ball according to claim1, wherein the golf ball has a compression deformation amount (shrinkingamount along the compression direction thereof) ranging from 2.0 mm to4.0 mm, when applying a load from an initial load of 98 N to a finalload of 1275 N to the golf ball.
 4. The golf ball according to claim 1,wherein the spherical core has a hardness difference (Hs—Ho) between asurface hardness (Hs) and a center hardness (Ho) thereof ranging from 15to 40 in Shore C hardness.
 5. The golf ball according to claim 1,wherein if a hardness is measured in the spherical core at nine points,including a core center and a core surface, obtained by dividing aradius of the spherical core into equal parts having 12.5% intervalstherebetween, and is plotted against a distance from the core center,coefficient of determination R² of a linear approximation curve obtainedfrom a least square method is 0.90 or more.
 6. The golf ball accordingto claim 1, wherein the spherical core has a compression deformationamount (shrinking amount along the compression direction thereof)ranging from 2.0 mm to 6.0 mm, when applying a load from an initial loadof 98 N to a final load of 1275 N to the spherical core.
 7. The golfball according to claim 1, wherein the spherical core is formed from arubber composition containing (a) a base rubber, (b) a co-crosslinkingagent and (c) a crosslinking initiator, and wherein particles formedfrom an α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or a metal salt thereof, or a crosslinked rubber powder are includedinside the formed spherical core.
 8. The golf ball according to claim 1,wherein the cover is formed from a cover resin composition having abending stiffness in a range from 500 kgf/cm² to 6000 kgf/cm².
 9. Thegolf ball according to claim 1, wherein the cover is formed from a coverresin composition having a slab hardness in a range from 35 to 65 inShore D hardness and having a ratio (slab hardness (Shore D)/bendingstiffness (kgf/cm²)) of the slab hardness to a bending stiffness in arange from 0.01 to 0.05.
 10. The golf ball according to claim 1, whereinthe cover is formed from a resin composition containing an ionomer resinand at least one filler selected from the group consisting of anorganically modified layered silicate, a carbon nanotube and acarbonaceous filler having a polar functional group on the surfacethereof.
 11. The golf ball according to claim 1, wherein the sphericalcore has a surface hardness ranging from 75 to 95 in Shore C hardness.12. The golf ball according to claim 1, wherein the spherical core has acenter hardness ranging from 35 to 65 in Shore C hardness.
 13. The golfball according to claim 7, wherein the crosslinked rubber powder has ahardness ranging from 15 to 65 in Shore C hardness.
 14. The golf ballaccording to claim 7, wherein the crosslinked rubber powder is includedin an amount ranging from 1 to 40 parts by mass with respect to 100parts by mass of (a) the base rubber.
 15. The golf ball according toclaim 7, wherein the particles formed from an α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms and/or a metal salt thereof have a modeparticle size of larger than 10 μm but at most 50 μm.
 16. The golf ballaccording to claim 7, wherein the particles formed from anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof include particles having a particle size ranging from6 μm to 300 μm in a volume ratio (V₆₋₃₀₀) of 60% or more and particleshaving a particle size of 200 μm or less in a volume ratio (V₀₋₂₀₀) of75% or more.
 17. The golf ball according to claim 7, wherein theparticles formed from an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or a metal salt thereof have a specific surface arearanging from 0.1 m²/g to 1.5 m²/g.
 18. The golf ball according to claim9, wherein the cover is a cover other than an outermost cover.
 19. Thegolf ball according to claim 18, wherein the outermost cover is formedfrom a cover resin composition having a slab hardness in a range from 10to 45 in Shore D hardness.
 20. The golf ball according to claim 10,wherein the resin composition contains the filler in an amount rangingfrom 5 to 60 parts by mass with respect to 100 parts by mass of theionomer resin.